EMBRYONIC DEVELOPMENT 205 



In the frog, whose gastrulation is of this type, the invagination appears 

 from the exterior as in Fig. 178. The cells are inturned along a short 

 crescent-shaped line, which becomes extended into a marked U, and 

 finally completes a circle which diminishes in size to a mere pore as the 

 yolk-filled cells are withdrawn inside. 



Gastrulation in strongly telolecithal embryos, like those of birds, 

 reptiles and most fishes, is so modified as to require an interpretation 

 o[ events too difficult for presentation here. In insects there is an 

 infolding which is usually called gastrulation, but the tissue turned in 



s 



J 



Fig. 176. — Earliest recognition of the distinction between somatic and germ cells in a 

 vertebrate animal. Diagram of cross section of the body of the embryo, showing germ cells 

 in the endoderm of the intestine and their path of migration (shown by arrows) to the site 

 of the reproductive organs. How much earlier than this stage the somatic cells have lost 

 their reproductive powers is not known, c, coelom; en, endoderm of intestine; gc, germ 

 cells; ge, germinal epithelium which later covers the gonads and from which the germ cells 

 issue; i, intestine; m, myotome, or muscle segment; ms, mesentery; nc, neural crest, from 

 which nerves and ganglia develop; nd, notochord, forerunner of the backbone; s, spinal cord. 



becomes not just the lining of the digestive tract but the whole internal 

 structure of the body. These two types are omitted from the com- 

 parisons in Fig. 177. 



Mesoderm Formation. — At the end of gastrulation at least two 

 layers of cells, ectoderm and endoderm, are present. In most multi- 

 cellular animals a third layer, the mesoderm, if not already present is 

 soon formed between these two. In the fishlike amphioxus, a classical 

 form in biology, the upper portions of the endoderm (Fig. 179) are 

 turned outward in the form of grooves, shown dotted in cross section 

 in the illustration {A). The edges of each groove meet and fuse, and 



