22 THE MOLLUSCA 
Limnaea, 20-100 eggs; felix, 40-100; Ancylus, 5 or 6; Valvata, 
17; 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. Empryo.oey. 
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, ete.). 
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 
uaavea Mma 
Fig. 9. 
A me 
ap 
owe) 
Eggs of various Mollusca, at the same stage, with 4 micromeres. A, Patella (Patten); B, 
Lacuna; C, Teredo (Hatschek); D, Sepia (Kolliker). ma, macromeres ; mi, 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 (7eredo, Fig. 9, C, Cyclas, Unionidae), in which there 
