120 



GASTROPODA. 



the mode of formation of this layer in Nassa as given by Bobretzkv 

 (No. 11) to the method described above. In sections made through 

 such a stage in the egg of Nassa (Fig. 42 E), under a cover of 

 smaller cells, a few larger cells can be seen projecting into the 

 cleavage-cavity. The projecting cells detach themselves and yield 

 a few somewhat large cells which from this time lie in the cleavage- 

 cavity. These are the first mesoderm-cells and, since the cells 

 from which they were abstricted evidently correspond to one of 

 the smaller macromeres (Fig. 12 E), the mesoderm has an origin 

 similar to that in the cases previously considered. The smaller 



tint, 



Fig. 48.— A-E, a few stages of the cleavage and formation of the germ-layers of 

 Umbrella (after Heymons). A shows the four macromeres ; B, the division of the 

 mesentomere ; C, the formation of the primitive mesoderm-cells ; I) and E, the 

 formation of the mesoderm-bands. I-IV, the four macromeres, or their derivatives. 

 ect, ectoderm ; ent, entoderm ; m, the primitive mesomere ; mes, mesoderm ; urn, the 

 paired mesoderm-cells (mesodermal teloblasts) resulting from the division of m. 



mesoderm-cells at first present soon again divide (Fig. 47 ^4), and 

 here also seem to yield structures akin to mesoderm-bands (Fig. 

 47 B). 



The mesoderm is also found to arise from the macromeres in various other 

 forms, e.g., in Limnaea (Wolfson, No. 131) and Fulgur (McMurrich, No. 

 70), and Janthina, in which form, according to Haddon (No. 40) it becomes 

 separated from the macromeres at the top of the blastopore. At a stage in 

 which the ectoderm-cap has not completely grown round the macromeres, the 

 peripheral macromeres yield the mesoderm-cells. Haddon's account is too 

 slight and his figures too vague to allow any conclusions to be arrived at 



