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Bashford Dean Memorial Volume 



a crescentic dark area which has very much the appearance of a cleft passing through 

 the blastoderm. The study of sections reveals that this dark area is really a cavity, the 

 segmentation cavity, covered by a thin transparent roof. As the blastoderm extends, this 

 dark area becomes less strongly marked and gradually disappears. (It has previously been 

 noted that a similar crescentic "dark" area occurs also in H. japonicus at a corresponding 

 stage, as evidenced by a sketch, without explanation, found among Dean's drawings. In 

 this sketch, dark and light areas are reversed, since the object was drawn, under low 

 magnification, by transmitted light). In the egg of H. pMlipi the light-yellow border, 

 previously mentioned, extends more rapidly than the blastoderm, and soon forms a broad 

 zone around the latter. It is quite evident that Has well does not consider the light' 

 yellow zone to be a part of the blastoderm; it is apparently the superficial expression 

 of the "parablast" (periblast). 



The internal structure of some early stages in the development of H. phillipi is the 

 chief topic of Haswell's article published in 1898. Beginning with a fairly early stage of 

 cleavage, the development of the germinal disc through the blastula and gastrula stages is 

 described and illustrated by figures drawn from sections. Two of these figures, represent' 

 ing early and late blastula stages, are reproduced here as my Text-figures 50 and 51. Their 

 resemblance to Dean's sections of corresponding stages of H. japonicus has already been 

 pointed out. 



GASTRULATION AND EARLY EMBRYOGENY 



In Elasmobranchs the changes that occur during gastrulation and early embryo 

 formation are complex, and cannot be adequately described without recourse to serial 

 sections. My only information concerning these stages in Heterodontus japonicus is 

 obtained from Dean's drawings of both opaque and cleared total embryos, and from one 



eci 



Text-figure 50. 

 Sagittal section of a blastoderm of Heterodontus phillipi in a stage showing the begin- 

 ning of the segmentation cavity (at the posterior end, to the right of and below 

 the segmented area in the figure). 

 ant, anterior; ect, ectoderm. 

 After Haswell, 1898, Fig. 1, pi. IV. 



