24 



LAMELLIBRANCHIA. 



2. Cleavage and Formation of the Germ-layers. 



In those forms in which the cleavage of the egg has been carefully 

 investigated {Unio, Anodonta, Gardium, Gydas, Teredo), its course 

 is so uniform that we may conclude that it is the same in those eggs 

 of which only a few but similar stages have been observed (Odrea 

 edulix, Pectm, Mytitus edulis, Mobius, Horst, Fullarton, Barrois 

 and Wilson). Cleavage is always unequal ; the first cleavage- plane 



divides the egg 

 into two cells, a 

 very large macro- 

 mere and a much 

 smaller micromere 

 (Fig. 11 A). In 

 Teredo, a corres- 

 ponding differen- 

 tiation is indicated 

 even before cleav- 

 age by the different 

 constitution of the 

 protoplasm at the 

 vegetative and at 

 the animal poles 

 of the egg. The 

 plane dividing the 

 two blastomeres passes through the point where the polar bodies 

 lie. The micromere next divides into two (Fig. 11 B), and almost 

 at the same time, or else a little later, the macromere gives origin 

 to a new micromere (G). The new cell then divides, and the process 

 of the abstriction of a micromere from the large cleavage-sphere is 

 repeated (D) again and again, the large cell yielding micromeres 

 which then divide (E). Finally, the micromeres, seen from the 

 surface, resemble a cap placed upon the remains of the macromere, 

 which at last also divides into two similar cells (macromeres) 

 (Fig. 11 F)* 



Fig. 11. — A-F, diagrams illustrating the cleavage of the 

 egg in the Lamellibranchia. The lines connecting the 

 nuclei of two cells indicate that the pair has arisen from 

 the division of one cell. 



* [It is commonly held that the entoderm arises solely from the macromeres 

 after the latter have ceased giving origin to micromeres, and that, at the four- 

 celled stage (Fig. 11 C), the rudiment of the entoderm lies entirely within the 

 single macromere. This appears to be the case in Cyclas according to 

 Stauffacher (No. VI.), but in Unio, on the contrary, Lillie (No. III.) 

 asserts that, at this stage, eacli blastomere contains the rudiments of both 

 ectoderm and entoderm. The cleavage in Unio appears, on superficial 



