Section 21. Hence, in Considering the Structure of Amphioxus, We

have three series of cells from which its tissues are developed:--

1. The epiblast.

2. Walls of the coelomic pouches, which form (a) an inner lining to the epiblast, (b) an outer coating to the hypoblast, and (c) the mesentery (m.), by which the intestine is supported. This is the mesoblast.

3. The lining of the mesenteron, or hypoblast.

From the epiblast the epidermis (not the dermis), the nervous system (including the nerves), and the sensory part of all sense organs are derived. From the mesoblast the muscles, the dermis genital and excretory organs, circulatory fluid and apparatus, any skeletal structures; and all connective tissue are derived. The mass of the body is thus evidently made of mesoblast. The hypoblast is the lining of the intestine and of the glands which open into it; and the material of the notochord is also regarded, as hypoblast.