CHAPTER XXXI. 

 SOLENOCONCHA (SCAPHOPODA). 



(DENTALIUM.) 



THE ontogeny of Dentalitnn was investigated many years ago 

 (1857) by LACAZE-DUTHIERS, and more recently (1883) by KOWA- 

 LEVSKY with the aid of sections ; the researches of KOWALEVSKY, 

 however, do not extend so far into the life of the animal as do those 

 of LACAZE-DUTHIERS, the former having been able to observe the 

 larva only up to the sixth or seventh day, while the latter was able 

 to keep the larvae alive until they were thirty-five days old. We 

 therefore still have to refer, for many points, to the older accounts of 

 LACAZE-DUTHIERS. 



The genital products are discharged into the water through the 

 right renal aperture, fertilisation taking place outside of the body. 

 The eggs, which are not very rich in yolk, are surrounded by a thin 

 envelope. 



1. Cleavage and Formation of the Germ-Layers. 



The cleavage is total, the egg dividing into two cleavage-spheres, 

 one of which is somewhat larger than the other. The larger sphere 

 then, by division, gives rise to a new sphere, and the smaller sphere 

 also divides into two, so that we have now one macromere and three 

 micromeres. It is possible that additional micromeres are segmented 

 oft' from the larger sphere. The former divide repeatedly, so that 

 there is soon a great number of -the micromeres lying upon a single 

 macromere which still remains rather large. This latter also finally 

 divides into two and then into four macromeres. This method of 

 cleavage shows considerable resemblance to that most common among 

 the Lamellibranchia. Further division and the formation of a 

 central cavity give rise finally to a blastula, one half of which con- 

 sists of small and the other of large cells (Fig. 34 A). The animal 



