264 



EMBRYOLOGY 



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

 remain more voluminous. In 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 cii'cle of cilia makes its 

 appearance, the future preoral ciliated band of the larva. 



tics -At- 



Fie. 117. — A, B, blastula and gastrula stages of Evpomatus vncinalus (after 

 H^tschbk); eh, egg-membrane; met, 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 enveloj^e (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 



