xxviii INVEETEBEATA 



FIG. PAGE 



334. Optical sagittal sections of two embryos of Callidina russeola after the 



process of gastrulation has been completed . . . . 423 



335. Ventral view of embryo of Callidina russeola somewhat older than 



those represented in Fig. 334 ...... 424 



336. Optical sagittal sections through two late embryos of Callidina russeola 



in order to show the formation of organs ..... 425 



337. Ventral views of two embryos of Callidina msseola at a stage not long 



before hatching . . . . . . . 426 



338. Optical sections of early embryos of Sagitta bipunctata . . 429 



339. Cross section of the blastula of Sagitta bipunctata in the 16-cell stage 



showing the determination of the mother cell of the genital organs . 430 



340. Optical section of older embryo of Sagitta bipunctata showing the forma- 



tion of the " head cavities " and of the stomodaeum . . . 431 



341. Ventral view of the larva of Sagitta enflata on the fourth day after hatching, 



in order to show the origin of the nervous system and the separation of 



the trunk coelom from the tail coelom ..... 432' 



342. The 2-cell stage of the egg of Ascaris megalocephala (monovalen's) when 



the spindles for the 4-cell stage are being formed, showing the diminution 



of the chromatin ........ 440 



343. Early stages in the segmentation of the egg of Ascaris megalocephala . 441 



344. The 12-cell stage of the segmentation of the egg of Ascaris megalocephala, 



seen from the left and the right sides respectively, in order to show the 

 rearrangement of the daughters of the cell AB .... 442 



345. Dorsal view of the segmenting egg of Ascaris megalocephala in the 20-cell 



stage ......... 442 



346. Ventral view of the segmenting egg of Ascaris megalocephala in the 24-cell 



stage ......... 442 



347. Dorsal view of the segmenting egg of Ascaris megalocephala in the 102-cell 



stage ......... 443 



348. Optical median sagittal section through the blastula of Ascaris megalo- 



cephala at the time that the process of gastrulation is commencing . 444 



349. Segmenting egg of Ascaris megalocephala in the 102-cell stage, seen from 



beneath ......... 445 



350. Sagittal sections through three embryos of Ascaris megalocephala in 



order to show the invagination of the genital cells and of the stomo- 

 daeal rudiment ........ 446 



351. Transverse section of embryo of Ascaris meyalocepliala in order to show 



the invagination of the anterior mesoderm cells and of the mother cells 



of the genital organs ..... 447 



352. A " T-giant " of Ascaris megalocephala, seen from the left side . . 449 



353. Two diagrams of the 4-cell stage in the development of the egg of Ascaris 



megalocephala in order to show the cytoplasmic zones which Zur Strassen 

 assumes to exist in the blastomeres ..... 450 



354. The stage corresponding to the 4-cell stage in the development of the 



normal egg which occurs in the development of the three types of doubly 

 fertilized eggs of Ascaris megalocephala ... 452 



355. Stage in the development of a "ball-egg" of Ascaris megalocephala cor- 



responding to the 8-cell stage in the development of a normal egg . 453 



356. Stages in the early development of Asterias vulgaris . 462 



357. Young larvae of Asterias mdgaris . . . 



358. Fully developed Bipinnaria larva of Asterias vulgaris, about three weeks old 465 



359. Larva of Asterias vulgaris, four days old, viewed from the dorsal surface, 



showing two madreporic pores 



