63 
is of phylogenetic significance, and indicates the origin of the 
' Class Lamellibranchs. 
The distinctive characteristics of this form of segmentation 
are the very small size of the eggs ; the appearance of bilate- 
ral symmetry with the first cleavage, and the indication at 
the same time of the anterior and posterior ends and right 
and left sides and dorsal and ventral surfaces of the adult ; 
the separation of the egg at the beginning of segmentation 
into a germinative portion, and a portion which is morpho- 
logically comparable, during the process of segmentation, to 
a food yolk, although it is less granular, both in the oyster 
and in Crenella, than the germinative portion, and at a later 
stage undergoes segmentation, and forms the wall of the diges- 
tive cavity, so that it has none of the physiological character- 
istics of a food-yolk; and the fact that many of the stages 
in the process of segmentation have no functional importance 
and may be suppressed. Outside the Lamellibranchs, when- 
ever we have very small simple eggs among the Mollusca, the 
embryo is radially symmetrical around a central axis, which 
passes through the polar globules, and it presents, during the 
process of segmentation, few points of resemblance to the egg 
of the oyster at the same period. In support of this state- 
ment, I may refer to Lapkester’s figures of the eggs of Hudi- 
branchs and Opisthobranchs (Developmental History of the 
Mollusca, Plates 5 and 9), to Fol’s figures of the eggs of Ptero- 
pods and Heteropods (Etudes sur development des Mollusques. 
Arch. d. Zool. -exp. et gen. 1875), to my own observations on 
the segmentation of the Pulmonate egg (Studies from the Bi- 
ological Laboratory of the Johns Hopkins University, 1879), 
and to Biitschli’s and Lankester’s accounts of the segmenta- 
tion of the egg of Paludina, (“ Entwickelungsgeschichte von 
Paludina vivipara.” Zeit. f. Wiss. Zool. 1877, and “ On the 
Coincidence of the Blastopore and Anus in Paludina vivi- 
para.” Quart. Mic. Journ. 1876.) 
Babl has briefly discussed the relation of the bilaterally 
symmetrical, irregular segmentation of the lamellibranchiate 
egg, to the regular axially symmetrical segmentation of the 
egg in most Gasteropods (Entwickelungsgeschichte der Map 
