Part 92

A study of the embryo entire, including the vascular area.

The blood islands show as irregular deeply stained masses in the vascular area. At this stage they are inclosed in wide anastomosing tubes, the extra-embryonic blood-vessels, which open peripherally into the bounding sinus terminalis.

The following structures may be identified:

The neural tube forming the axis of the embryo. In the anterior region may be noted the forebrain, the optic vesicles, the midbrain. The hindbrain is subdivided into the neuromeres. The cord, or myelon, of the neural tube back of the hindbrain is closed in front but is open behind.

The head projects above the blastoderm. The fold which unites the ventral surface of the head with the blastoderm is called the head fold.

The diverging folds of the myelon encloses the primitive streak.

On each side of the neural tube are the mesoblastic somites. The series is continued behind by the undivided segmental plate.

The heart is beneath the hindbrain. The portion of the body cavity in which it lies, is bounded in front by the head fold and behind by the diverging limbs of the splanchno-pleure. Its posterior or venous end receives the vitelline veins from the vascular area. The anterior or arterial end is prolonged into the ventral aorta.

Note that the axis is somewhat bent.

The head fold of the amnion is noted to extend over the anterior end of the head.

A study of the transverse sections.

The sections about 20 micro-millimeters thick should be cut serially, and so mounted. Knowing how many micro-millimeters the embryo is in length will enable one to make a diagram of the fetal structure in the study of the series of transverse sections. The sections should be drawn in the order studied so as to obtain relative structural ideas.

With the microscope study the following regions:

1. Optic vesicle.

2. Midbrain.

3. Posterior half of the heart.

4. Myelon in the closed region.

5. Myelon in the open region.

6. Through the primitive streak.

2. THE EMBRYO TWENTY TO TWENTY-FOUR HOURS OLD WITH FROM TWO TO SIX SOMITES.

Compare the parts in this embryo with those of the embryos from twenty-nine to thirty-four hours old. Note and describe the relations of the embryo, the area pellucida, and the area opaca. Note how much of the yolk the blastoderm covers.

In studying the entire embryos, note the condition of the medullary plate. Observe if the tube is formed in any part; how far back the head plate extends, if the heart can be seen; and if the primitive streak is longer or shorter than in 1.

Study relations of structures and make drawings through the point of divergence of the walls of the fore gut; also through the somatic region, and through the primitive streak.

Study all parts, as the head fold, the heart, and the fore gut.

3. THE EMBRYO FORTY-FOUR TO FORTY-EIGHT HOURS OLD WITH TWENTY-FOUR TO TWENTY-NINE SOMITES.

Remove the embryo with the entire area, and preserve it.

Note and carefully describe the changes visible to the naked eye since the thirty-fourth hour.

In making a study of the entire embryo we note there has been a rapid growth of the dorsal surface of the head, which has become more bent. This bend, in the region of the midbrain is called the cephalic flexure. The forebrain and part of the midbrain form almost a right angle with the rest of the head. The head is compressed laterally and free from the blastoderm. The dorsal side of the trunk is turned up, and there is a twisting of the axis of the embryo just back of the heart. The tail fold begins at about this time and may or may not be visible. The optic vesicles are relatively smaller in relation to the brain than in 1. Note the part of the forebrain to which they are attached. Observe the inner and outer layers of the retina, the lens, the choroid fissure, and the cavity of the vitreous humor.