ILLUSTRATIONS xxv 



FIG. PAGE 



248. Horizontal section through the visceral hump of an older embryo of 



Paludina vivipara than that represented in Fig. 247 in order to show the 

 formation of the kidneys and the heart ..... 313 



249. Illustrating the development of the ureters of Paludina vivipara and their 



relation to the kidneys ....... 314 



250. Two embryos of Paludina vivipara, viewed from the right side, in order to 



show the origin of the sense-organs and the beginning of torsion . .316 



251. Just hatched Paludina vivipara, seen from the left side and viewed as a 



transparent object ........ 317 



252. Transverse sections through the visceral humps of two embryos of Paludina 



vivipara of different ages in order to illustrate the torsion of the organs, 

 the development of the genital organs and their connexion with the right 

 kidney, and the lengthening of the visceral hump associated with an 

 increase in length of the liver . . . . . .318 



253. Apical region of an embryo of Crepidula showing the Molluscaii cross in 



a late stage of development ...... 322 



254. Embryo of Limnaea stagnalis, viewed from right side as a transparent 



object, in order to show the larval kidney ..... 324 



255. Vertical sections of the eggs of Dentalium before and after fertilization in 



order to show the flow of cytoplasmic substances .... 326 



256. Stages in the cleavage of the egg of Dentalium .... 327 



257. Further stages in the cleavage of the egg of Dentalium . . . 328 



258. The Trochophore larva of Dentalium, twenty-six hours after fertilization . 329 



259. Transverse section of the Trochophore larva of Dentalium in the region of 



the prototroch ........ 329 



260. Veliger larvae of Dentalium ....... 330 



261. Larvae resulting from the development of eggs of Dentalium from which 



the polar lobe has been removed ...... 331 



262. A r ertical section of a larva of Dentalium developed from egg from which 



the first polar lobe had been removed in order to show the absence of 

 meso'derm ......... 331 



263. Longitudinal section of the 2-cell stage of Dreissensia polymorpha in order 



to show the blastocoele ....... 333 



264. Stages in the cleavage of the egg of Dreissensia polymorpha . . 334 



265. Sagittal sections of embryos of Dreissensia polymorpha, showing the process 



of gastrulation and of the formation of the shell gland . . . 336 



266. Young Trochophore larva of Dreissensia polymorpha, seen from the ventral 



side .......... 337 



267. Sagittal section through a young Trochophore larva of Dreissensia poly- 



morpha ......... 338 



268. Transverse section of the ventral portion of a young Veliger larva of 



Dreissensia polymorpha in order to show the origin of the mantle-groove 



and of the pedal ganglia ....... 338 



269. Young Veliger larva of Dreissensia polymorpha, seen from the side . 339 



270. Young Veliger larva of Dreissensia polymorpha, seen from the ventral 



surface ......... -340 



271. Older Veliger larva of Dreissensia polymorpha, seen from the side. This 



stage is the one which immediately precedes the metamorphosis . . 341 



272. Transverse section through the dorsal region of an old Veliger larva of 



Dreissensia polymorpha in order to show the differentiation of the 

 pericardium and the kidneys ...... 342 



273. Sections through the cerebral pit of Veliger larvae of Dreissensia poly- 



morpha ........ 343 



