248 bulletin: museum of comparative zoology. 



the very intense blue color which they take upon treatment with a 

 hematoxylin stain. Their cytoplasm is not homogeneous at this stage, 

 but contains numerous large dark-looking granules. The granules are 

 undoubtedly yolk granules, and their dark appearance can often be seen 

 upon close inspection to be due to an enveloping film of deeply staining 

 protoplasm, which often extends out in radial processes, giving the whole 

 a star-like appearance. This I believe to be caused by the progressive 

 assimilation of the yolk granules and their conversion into protoplasm. 

 In the case of the mitotic cell J. 7-6 (Fig. 67), and likewise of its mate in 

 the left half of the same Figure, the characteristic mesenchyme stain- 

 ing appears only in the more superficial portion of the cell, its deeper 

 portion being loaded with yolk granules, which are still almost unat- 

 tached by the protoplasm. Consequently, when the approaching divis- 

 ion is accomplished, the sister cells formed will differ from each other 

 in appearance, the more superficial one being deeply stained, the other 

 being stained scarcely at all. Subsequently, however, the yolk-laden 

 cell will come to resemble in appearance its sister cell, and will have 

 the same ultimate fate. The nuclei of the mesenchyme cells resemble 

 closely in appearance those of the endoderm cells. In the case of c 7,12 

 and d 7A ' 2 (Figs. 65 and 66), the nuclei are relatively small on account 

 of recent division. 



The eight anterior chorda cells (a ! - u , Fig. 68; a 7 - 9 , a 7 - 10 , and a 7 - 13 , 

 Fig. 69 ; together with the corresponding cells in the left halves of these 

 Figures) resemble closely in shape and stainability the endoderm cells. 

 They are smaller, however, and contain nuclei, likewise smaller, with 

 less conspicuous chromatic granules (omitted altogether in the Figures, 

 as previously explained, to aid in readily distinguishing the chorda cells 

 from those of other organs). 



The two posterior chorda cells (d 7A \ Fig. 66 ; C 7A1 [by mistake of 

 engraver for c 7 - 11 ], Fig. 67) stain more deeply than the anterior chorda 

 cells, resembling to some extent their sister cells d 712 and c 7 ' 2 (Figs. 65 

 and 66), from which they have recently been separated by division. 

 However, they are many times smaller than their sister cells, and extend 

 less deeply. This difference is connected with the oblique position of the 

 spindles in the mother cells (see d Re , Fig. 60), a matter to which atten- 

 tion was called in the discussion of the 64-cell stage. 



In the neuro-muscular ring the cells (stippled to distinguish them 

 from those of other groups) have about the same histological character in 

 both anterior (^ 8 - 16 , Fig. 68 ; A H - 15 , Fig. 69; A s7 , A™, Fig. 70 ; together 

 with the corresponding cells in the left halves of these Figures) and pos- 



