DEVELOPMENT 269 



larger pale, yolk -laden cells below the equator. As division pro- 

 ceeds, soon there are many cells arranged in the form of a hollow 

 sphere which will be recognized as the blastula. (Fig. 174.) 



The transformation, in due course, of the blastula into the 

 gastrula by typical invagination is somewhat obscured by the 

 large amount of yolk in the prospective endoderm cells. In fact, 

 the endoderm is formed by a flat infolding of cells, just below the 

 edge of the small dark cells, that finally results in a crescentic 

 groove on the surface, which is the edge of the blastopore. But 

 gastrulation is not completed until all the large endoderm cells 

 are enclosed by ectoderm cells, and this is accomplished chiefly 

 by the latter gradually creeping and folding over the exposed, 

 pale endoderm cells (yolk plug) until merely a small blastopore 

 remains leading into the enteron. 



The development of the third germ layer, or mesoderm, takes 

 place by the ingrowth of a layer of cells between ectoderm and 

 endoderm along the edge of the blastopore where these two layers 

 merge. When the mesoderm has grown forward and spread out 

 between the ectoderm and endoderm, the lower portion (lateral 

 plate) splits into a somatic and splanchnic layer and thus gives 

 rise to the coelomic cavity between. The upper portion of the 

 mesoderm (vertebral plate) exhibits traces of primitive seg- 

 mentation as it forms a series of muscle plates, or myotomes, on 

 either side of the notochord which, in the meantime, has arisen 

 from an axial, dorsal strip of cells above the enteron. 



During the later stages of gastrulation, the foundations of the 

 central nervous system are established by the differentiation of a 

 plate of ectoderm cells, the medullary plate, on the dorsal sur- 

 face of the embryo. Then a groove appears in the medullary plate 

 which is finally completely enclosed by the growth upward, over, 

 and then fusion of the edges of the medullary plate. Thus the 

 open groove is converted into the neural tube which is soon to be 

 differentiated into fore-brain, mid-brain, hind-brain, and spinal 

 cord. 



Simultaneously with the establishment of the central nervous 

 system, the differentiation of the enteron into the alimentary canal, 

 opening by mouth and anus, and also various other internal trans- 

 formations have proceeded apace. Furthermore, the embryo has 

 been gradually elongating so that it is nearly twice as long as 

 broad by the time it begins locomotion by means of cilia distributed 



