CLEAVAGE AND FORMATION OF THE GERM-LAYERS. 



Ill 



The formation of the germ-layers does not take place in all Gastro- 

 pods in the manner just described, indeed, the layers form in very 

 different ways in diverse Gastropods, as might be expected from the 

 variations found in the manner of cleavage. It has already been 

 mentioned that such variations occur in spite of strong general 

 resemblance. The method of cleavage described above applies, with 

 slight modifications, to many Gastropods. We append a list of a few 

 genera chosen as representatives from the different divisions in which 

 this is the case: among the Prosobranchia, Fissurella (No. 12), 

 AV/vV/w/ (No. 7), Crepidula (Nos. 24 and 25), Eythinia (Nos. 91, 

 101 and 28), Vermrtu* (No. 99), Fusm (No. 11), Entoconcha (No. 76) ; 

 among the Heteropoda, Firoloida and Pterotrachea (No. 31) ; among 

 tlu- Pulmnmita, Planorbi* (No. 91), Limnaea (Nos. 130 and 131), 



|-'i.. 11. St.-i.m-s in the cleavage of Cavolinia tridentata (A) and Aplysia limaciua (B) 

 (after FOL and BLOCHMANN). 7.-/T r ., the four macromeres, above them lie the 

 niii-ronirres and the polar bodies (rk). 



Limn.'- (Xos. 130 and 73), Onchidium (No. 51) ; among the Opistho- 

 branchia, Doto, (No. 91), Ercolania (No. 124), Tethys (No. XXVI), 

 Umbrella (No. XII) ; among the Pteropoda, Cavoliniii, Cymbulia 

 (No. 32), Clione (No. 55). 



Certain modifications in the cleavage are no doubt principally 

 determined by the amount of yolk in the egg. These are connected 

 specially with the size of the macromeres. In Cavolinia and Cymbuliu^ 



are correct, then there must be two anterior and two posterior macromeres, 

 and it is from one of the latter that the primitive mesomere is now said to 

 arise. It seems further probable that the first mesomere arises from the left 

 posterior macromere in dextral and from the right in sinistral Gastropoda. In 

 spite, however, of the large amount of evidence which is accumulating in favour 

 of this view we must, when we consider the great difficulty in tracing the rela- 

 tions of the early cleavage-planes, wait for further observations, especially on 

 Prosobranchs, before we finally conclude that this origin of the mesoderm is 

 typical of all Gastropoda. See footnote, p. 119. ED.] 



