192 



INVEETEBEATA 



CHAP. 



place by which an archenteron is formed which is large and occupies 

 most of what was the interior of the blastula. 



The development of the Schizopod Euphausia, as far as the 

 gastrula stage, has recently been worked out by Taube (1909). 

 Here, as in Lucifer, the egg undergoes total segmentation and the 

 blastomeres are all of nearly the same size. In this way a hollow 

 blastula is formed. After the 32-ceU stage, however, the ceUs do not 

 all divide ; two remain undivided, and form the rudiment of the endo- 

 derm, and these, at the 112-cell stage, pass into the interior of the 



Fig. 141. — Sections through the developing egg of Mysis chamaeUs. {After Wasbaum.) 



A, ventral part of egg showing the solid ingrowth of cells which replaces invagination. B, trans- 

 verse section of embryo showing the formation of the epithelium of the mid-gut. etld, endoderm ; hep, 

 liver saccule ; v. tic, ventral nerve cord. 



blastula. The blastopore is surrounded by a ring of special cells, 

 and of these two are said to give rise to the mesoderm. 



In Mysis and its allies, and in Amphipoda and Isopoda, in a word 

 in all the Peracarida, however, the invagination is replaced by a sohd 

 ingrowth of endoderm cells, and when these detach themselves and 

 wander through the yolk, they form an endodermal epithelium, at 

 first only on the ventral side of the yolk (Pig. 141). Only very 

 gradually does this epithelium extend so as to enclose the yolk on 

 the upper side also. In these cases . too we have meroblastic seg- 

 mentation, i.e. the zygote nucleus, whether it is in the interior of 

 the egg, as in the Isopoda, or on the surface as in Mysis, gives rise to 



