228 EDWIN G. CONKLIN. 



PHOTO 9. Four-cell stage from vegetal pole, showing the yellow crescent across 

 the two posterior cells. The anterior cells lie at a lower level than the posterior ones 

 and the focus is such that only the ends of the crescent show clearly. 



PHOTO 10. Four-cell stage from the animal pole, high focus ; many test cells cover 

 the egg ; the yellow crescent, which lies on the lower side of the posterior cells, 

 shows indistinctly through the egg ; an area of clear protoplasm is shown in each of 

 the cells. 



PHOTO ii. Eight-cell stage from the right side ; the upper cells contain the clear 

 ectoplasm, though a small amount of yolk is found at the periphery of each cell ; 

 most of the yellow protoplasm is contained in the yellow crescent, the outline of 

 which is very distinct, but a small amount of yellow protoplasm is found around the 

 nuclei of all the cells ; in the posterior ventral (upper) cells this lies on the lateral and 

 dorsal side of the nucleus, in the anterior dorsal (lower) cells it lies on the 

 lateral and posterior side of the nucleus. At the middle of the yellow crescent and 

 seen as a notch in its posterior outline is a small cap of caudal chymoplasm (the same 

 as that seen in Photo 3). The yellow crescent is bounded by dark gray endoplasm 

 which extends forward to the middle of the anterior-dorsal cells ; the gray crescent 

 of chorda- neuroplasm occupies the anterior portions of these cells. The forward 

 slant of the vertical (second cleavage) furrow and the " cross furrow' 1 formed by it 

 and the third cleavage are clearly shown. Photos 3, 8, and II are all viewed from 

 the right side and the localizations of the same organ-forming substances in the I-, 2- 

 and 8 cell stages are clearly shown in these photos. 



PHOTO 12. Sixteen-cell stage from dorsal side, view slightly oblique. The eight 

 dorsal cells are clearly shown, while three transparent ventral cells are indistinctly 

 shown on the left-anterior periphery. The yellow crescent is contained in the four 

 posterior cells, the lighter margins of the four anterior cells represent the gray 

 crescent. The median cells behind are nearly filled with yellow mesoplasm, save 

 for the clear nuclei and a small wedge of gray substance in the anterior portion of 

 these cells ; the lateral portions of the cells just anterior to these are composed of 

 yellow substance, their median portions of gray material. 



PHOTO 13. Sixteen-cell stage from the posterior pole, showing the four yellow cres- 

 cent cells with clear nuclei ; below them is the gray endoplasm, above them the clear 

 ectoplasm ; the ectoderm cells are indistinctly shown with a trace of yellow substance 

 around nuclei of the four posterior cells. 



PHOTO 14. Sixteen-cell stage transitional to 32-cell stage, dorsal view showing 

 eight cells. The localization of the different ooplasmic substances is the same as in 

 Photo 12, but the focus is a little deeper. The yellow and gray crescents are remark- 

 ably distinct ; between the two is the area of deep gray endoplasm ; the light area on 

 the inner border of the yellow crescent is chymoplasm. 



PHOTO 15. Thirty -two cell stage, dorsal view. The small posterior crescent cells 

 have divided transversely, forming four small cells ; the large mixed cell anterior to 

 these is just cutting off its outer yellow portion from its inner gray one. The four 

 anterior cells have divided in an antero-posterior direction thus separating the gray 

 crescent of chorda- neuroplasm from the endoplasm. The nuclei in all the cells ap- 

 pear as clear areas. 



PHOTO 1 6. Thirty-two cell stage, dorsal view, similar to the preceding. 



