250 



ZOOLOGY. 



From this point where the embryo is destined to be developed, 

 a sheet of cells gradually extends itself by growth around the 

 yolk. Thus a yolk-sac is formed by means of which the food 

 is used in the further development of the embryo. By the time 

 the embryo is hatched the yolk is exhausted. Although the 

 yolk does not segment we see that it serves its purpose in the 



FIG. 113. 



mi. 



mes.- rtr^ 



ma. 



FIG. 113. Diagram of early segmentation stages in a Gasteropod. A, 2-celled stage; 

 B, 4-celled; C, 8-celled; D, later stage, in section, ect., ectoderm cells (micromeres) ; 

 ent., entoderm cells, macromeres; tncs., mesoblasts, early put aside, before gastrula- 

 tion to form the mesoderm; mi., micromeres; ma., macromeres. 



Questions on the figures. What causes are assigned for the differ- 

 ence in the size of the cells in the 8-celled stage ? In what other ways is 

 mesoderm formed in the metazoa ? Which cells seem to divide more 

 rapidly, the micromeres or the macromeres? Compare with Annelid, 

 Fig. 103. 



development of the embryo. The later development is typi- 

 cally indirect, i. e., with a metamorphosis, though many (as 

 the cephalopods) develop directly into the adult form. A 

 larval stage (trochosphere} occurs, suggesting the larvae of 

 the Polychseta. This is followed by another stage (veliger} 

 which is more characteristic of the Mollusks. 



293. Library Exercises. Appoint students to supplement the text by 

 making short reports on the following topics : the early segmentation of 

 lamellibranchs and gasteropods ; of the cephalopods ; the veliger of 

 mollusks ; the formation of the organs in cephalopods ; development in the 



