730 



Bashford Dean Memorial Volume 



Text-figure 48. 

 Late stages in the reconstruction of a "resting" nucleus in the blastoderm of Heterodontus japonicus. 

 In A, the daughter nucleus is composed of chromosomal vesicles — some fused into larger vesicles. 

 In B, the process of fusion is almost complete, but maternal and paternal components are segregated 

 in two distinct groups. In C, the duplex character of the nucleus is indicated only by a notch 



on one side. 



From drawings left by Bashford Dean. 



compared with those that immediately precede and follow it, one gets the impression that 

 the pale-yellow zone also is a part of the blastoderm. On this view, it becomes easier to 

 explain the round dark spot at the posterior (lower) end of Figure 80, plate VII: it may be 

 derived from the crescentic area (dark when viewed by reflected light) which is the optical 

 expression of the segmentation cavity. But one notes, in this figure, that the posterior 

 and lateral margins of the reddish area are slightly upraised, which is fairly convincing 

 evidence that this area alone constitutes the blastoderm. If we consider the blastoderm 

 to be confined to the reddish area, then the dark spot may be simply an oil globule. 

 Probably, the figure under consideration was drawn, under low magnification, as seen 

 through a thick layer of albumen. 



It does not lie within the scope of this article, as indicated by its title, to consider the 

 internal development. Nevertheless, it seems desirable to bring together the scanty 

 available data concerning the early stages. The few drawings made from microscopic 

 sections, found among Dean's records, seem of sufficient historical interest to justify their 

 inclusion here. These drawings (Text-figures 46 to 48) are concerned with mitosis 

 during cleavage. The originals, probably drawn by Dean himself, lack explanations or 

 labels save the words "'Cestracion blastomeres" written below Text-figures 46a, 47b and 

 48b; also the words '"'Cestracion — budding of blastomeres" under Text-figure 47a. These 

 explanations are in Dean's handwriting. The drawings are not dated, but the paper is 

 yellow with age. In the light of our present knowledge, the transformation of chromosomes 

 within chromosomal vesicles is clearly portrayed in Text-figures 46c and 47a. Chromo- 

 somal vesicles of a single daughter nucleus are shown in Text-figure 47b. Fusion, in 

 varying degrees, of chromosomal vesicles derived from the same parent is shown in Text- 



