264 



EMBRYOLOGY 



at the vegetative pole, and thus the blastomeres at the latter 

 remain more voluminous. Jn the resulting blastula, the cells 

 from which the three germ-layers arise are already differen- 

 tiated (Fig. 117 A). The upper hemisphere, composed of 

 smaller cells, gives rise to the ectoderm, and the greater 

 part of the lower to the entoderm ; however, two cells here 

 are distinguished at an early period from the others by 

 assuming a more spherical shape : they produce the mesoderm, 

 and are called by Hatschek the primitive mesoderm cells 

 [mesodermal teloblasts (Fig. 117)]. The region where they 

 lie corresponds to the anal end of the larva. Even as early 

 as this stage a delicate equatorial circle of cilia makes its 

 appearance, the future preoral ciliated band of the larva. 



nar 



Fig. 117. — A, B, blastula and gastrula stages of Evpomatus nncinatus (after 

 Hatschek); eh, egg-membrane; mes, one of the two mesoderm cells. 



Soon afterwards the tuft of cilia at the apex of the larva 

 makes its appearance (Fig. 117 B). The cilia perforate 

 the egg-membrane, which therefore most probably consists 

 of a soft mass. 



The subsequent behaviour of the egg-membrane is of a peculiar nature 

 for, according to the concurrent statements of various authors, it is pro- 

 visionally retained, increases in extent with the growth of the larva, and 

 is thus formed into a cuticula-like envelope (Hatschek, No. 20), which, 

 however, is replaced later by the permanent cuticula from the ectoderm. 

 Thus here the embryo is converted directly into the larva. 



The entodermic part of the blastula now invaginates. At 



