MOLLUSCA 



255 



invagination of the large cells or by the overgrowth of the 

 micromeres, depending on the size of the segmentation cavity 

 and of the entodermal cells. In the cephalopods, owing to the 

 large supply of food substance in the ovum, cleavage is confined 

 to a small disc at the active pole. 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 de- 

 velopment 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 development of the embryo. 



Pig. 115. 



mes.— iSr^? 



ent. 



ma. 



Fig. IIS. Diagram of early segmentation stages in a Gasteropod. A, a-celled stage; B, 4- 

 cetled; C. S-celled; D, later stage, in section, tci.^ ectoderm cells (micromeres) ; en(.. entoderm cells, 

 macromeres; mes., mesoblasts, early pat aside, — before gastrulation — to form the mesoderm; mi., 

 micromeres; ma., macromeres. 



Questions on the figures. — What causes are assigned for the difference 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, Pig. 105. 



The later development is typically indirect, i.e., with a metamor- 

 phosis, 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 moUusks. 



299. Library Exercises. — Students may well 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 



