.22 



THE MOLLUSC A 



Limnaea, 20-100 eggs; Helix, 40-100; Ancylus, 5 or 6 ; Valvata, 



1 7 ; Neritina, 50-60. Lastly, the number of eggs is small in 



viviparous forms : in Paludina there are about 15; in Subulina 

 {Fig. 8) there are 4. 



III. EMBRYOLOGY. 



In the Mollusca the segmentation of the egg is unequal. In 

 some primitive forms, it is true, the first two or three divisions are 

 regular (Patella, Fig. 9, A; Chiton), and the two first blastomeres 

 are often equal in size, but it is frequently the case that even the 

 first division is unequal (many Lamellibranchs, Rachiglossa, etc.). 

 From the first phases of segmentation up to the blastula stage one 

 may distinguish two kinds of cells or blastomeres : the ectodermic 

 cells, called ectomeres or micromeres ; and the endodermic cells, 

 called endomeres or macromeres. In the more primitive forms, 

 however, such as Patella (Fig. 9, A), and the Protobranchia, there is 



FIG. 9. 



Eggs of various Mollusca, at the same stage, with 4 micromeres. A, Patella (Patten) ; B, 

 Lacuna ; C, Teredo (Hatschek) ; D, sepia (Kolliker). ma, macromeres ; /;(/, micromeres. 



scarcely any difference in size between the two kinds of blastomeres, 

 but in proportion as the endodermic cells are charged with food-yolk 

 so does the difference in size become accentuated, as may be seen 

 in the Taenioglossa (Fig. 9, B), and particularly in the Lamellibranchs 

 (Fig. 9, C) and Rachiglossa. In the last-named group the irregu- 

 larity of segmentation reaches its maximum, leading to the stage 

 of incomplete or meroblastic segmentation characteristic of the 

 Cephalopods (Fig. 9, D). It is quite exceptional for the segmenta- 

 tion to become secondarily regular, as in Paludina, where this 

 condition is due to the diminution of the yolk (Fig. 110). 



The separation of the ectodermic from the endodermic elements 

 of the embryo follows a constant rule, similar to that which obtains 

 among the Annelida. After the formation of the four primary 

 micromeres two new series or quartettes of micromeres are formed 

 at the expense of the large endodermic cells, and all the ectodermic 

 organs are formed from these three quartettes. The micromeres 

 multiply more rapidly than the macromeres, and there are even 

 cases, such as Dentalium (Fig. 184), certain Rachiglossa, and 

 Lamellibranchs (Teredo, Fig. 9, C, Cyclas, Unionidae), in which there 



