xxvi INVERTEBRATA 



FIG. PAGE 



274. Horizontal section through the anterior portion of an old Veliger larva of 



Dreissensia polymorpha in order to show the differentiation of the 

 cerebral ganglia from the cerebral pit ..... 344 



275. Side views of two young specimens of Dreissensia polymorpha after the 



metamorphosis has taken place ...... 345 



276. Horizontal sections through the anterior portions of three just metamor- 



phosed specimens of Dreissensia polymorpha in order to show the trans- 

 formation of the cerebral pit into the labial palps .... 346 



277. Diagrammatic transverse sections of young specimens of Dreissensia poly- 



morpha in order to illustrate the development of the kidney . . 347 



278. Transverse section through a young Dreissensia polymorpha in order to 



show the origin of the genital organs ..... 348 



279. The late Veliger larva of Ostrea virginiana, viewed from the side . . 349 



280. The Veliger larva of Yoldia limalula, about three days old . . 350 



281. Longitudinal sagittal section through the Veliger larva of Yoldia limulata, 



three days old ........ 350 



282. Diagrams illustrating the development of the pericardium in Cyclas and 



Dreissensia . . . . . . . . .351 



283. Glochidium larva of Margaritana with widely opened valves, viewed as a 



transparent object from the dorsal surface ..... 352 



284. Transverse section of a Glochidium larva of Unio which is already fixed 



in the tissues of its host ....... 353 



285. Segmenting egg (32-cell stage) of Sepia officinalis, viewed from the animal 



pole . . . . . . . . . .355 



286. A portion of the margin of the blastoderm of the egg of Sepia officinalis 



at the conclusion of the process of segmentation in order to show the 

 transformation of the blastocones into cells of the yolk-membrane . 356 



287. Two sections through the edge of the blastoderm of Sepia officinalis in 



different stages of development in order to illustrate the development 



of the lower layer cells . . . . ... 357 



288. Two early embryos of Loligo vulgaris ..... 357 



289. Embryo of Loligo vulgaris, seen from the posterior side at the conclusion of 



the period of development, termed by Faussek Stage I. . . . 358 



290. Two sagittal sections of early embryos of Loligo vulgaris in order to 



illustrate the first formation of organs ..... 359 



291. Embryo of Loligo vulgaris, seen from the side and behind, in order to 



illustrate the development of the ganglia. The embryo is younger than 



that represented in Fig. 289 ...... 360 



292. Embryo of Loligo vulgaris in the stage termed by Faussek Stage 2, when 



the embryo begins to be grooved off from the yolk-sac ; posterior view . 360 



293. Sagittal sections of two embryos of Loligo vulgaris to illustrate the forma- 



tion of internal organs ....... 361 



294. Two diagrammatic transverse sections through a young embryo of Loligo 



vulgaris in order to illustrate the origin of the coelom and the blood 

 cavities ......... 362 



295. Diagrams of three transverse sections through an embryo of Loligo vulgaris 



much older than that represented in Fig. 294 .... 363 



296. Embryo of Loligo vulgaris at the period when the funnel is formed, viewed 



from behind ........ 364 



297. Transverse sections of embryos of young cuttle- fish (illustrating two stages 



in the development of the genital organs ..... 365 



298. Two early stages in the development of the eye of Loligo vulgaris, seen in 



transverse section ........ 366 



