728 Bashford Dean Memorial Volume 



the appearance of an egg of Lepidosteus (Eycleshymer, 1899), or of the amphibian Crypto^ 

 hranchus (Smith, 1912), in an advanced stage of cleavage. In nearly every drawing of the 

 egg of Heterodontus japonicus in the stages under consideration there is indicated, in 

 addition to the "cleavage" pattern just described, a very small circular germinal disc 

 similar to that of other Elasmobranchs. The germinal disc is usually situated a few 

 degrees above the equator. From Dean's note and manuscript it appears that this con' 

 ventional germinal disc (described as reddish in H. japoyiicus, reddish-yellow, orange- 

 yellow or simply orange in other Elasmobranchs) is already cut up into blastomeres 

 (Figures 7 and 8, plate I). In a preliminary sketch, found in Dean's notebook, of the egg 

 represented in Figure 5, plate I, the small circular area is labelled ''''h''d''m''' (blastoderm). 



The most puzzling thing about the cleavage of the egg of Heterodontus as described 

 by Dean is that there are apparently two distinct centers of blastomere formation. If 

 there are really two, the relationship between them is not clear. On this topic Dean 

 (1901.1, p. 4) comments as follows: "There is evidence that the present position of the 

 germ disc is a secondary one, for in eggs just deposited, (1) it is nearer the animal pole 

 than in later stages; (2) there is a kind of track, whitish in color, extending from the 

 direction of the upper pole of the egg, suggesting therefore that the disc has shifted its 

 position, leaving a wake behind". Dean (1901.1, p. 7) writes further: ''Cestracion 

 (Heterodontus) also indicates that the change in the position of the germ disc occurred 

 before holoblastic cleavage was given up, and we have with it a suggestion that it was 

 from some new or modified physiological cause that a distinction came to arise between 

 the germ disc and the region of the upper pole." Certain it is that the tiny germinal disc 

 soon takes the lead in the formation of the embryo. 



DISCOID AL CLEAVAGE AND THE BLASTULA 



Figure 79, plate VII, in color, shows the general appearance of the egg at the begin- 

 ning of the stages about to be described. In this egg, the furrows traversing the general 

 surface assume a pattern that is not the most fortunate to illustrate Dean's thesis that they 

 are cleavage furrows, but the drawing is the best one available to portray the colors as 

 described in Dean's notes. In his notebook Dean has written, concerning this early stage, 

 that the uppermost portion of the egg is "light" (yellow) and the equatorial region is 

 "greenish" (yellow). The region below the equatorial zone is designated simply" yolk". 

 In the drawing, the germinal disc appears pink (Dean calls it "reddish"), and it is surround- 

 ed by a white zone — repeatedly mentioned by Dean in his notes. 



Attention must now be focussed on the progress of segmentation within the germi- 

 nal disc. The very early stages of cleavage in the germinal disc of Heterodontus japonicus 

 have not been described. These stages must occur while the egg is still in the oviduct, 

 before or during the formation of the capsule. The earliest stages obtained by Dean are 

 those illustrated in Figures 7 and 8, plate I. These eggs were taken from the body of the 

 fish; they were enclosed in capsules that were practically complete, and were soon to be 

 deposited. A later stage is portrayed in Figure 9, plate I. In this drawing the blasto- 



