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Bashford Dean Memorial Volume 



Text'figure 53. 

 Sagittal section of a blastoderm oi Heterodontus philUpi in a stage in which gastrulation has just begun. 



ant, anterior; ect, ectoderm; end', parablast or periblast entoderm. 

 After Haswell, 1898, Fig. 6, pi. V. 



The blastula represented in my Text-figure 51 (after Haswell) is not ready for gastru- 

 lation. Before gastrulation begins, the blastoderm increases somewhat in diameter, and the 

 segmentation cavity (or blastocoele) extends throughout almost its entire length. The 

 floor of this cavity consists of a layer of yolk with unusually fine granules, unsegmented 

 but containing nuclei. Haswell (1898) refers to this layer as the '"parablast", but it is 

 evidently the "periblast" or "yolk syncytium" of other authors (e.g., Ziegler, 1902). As 

 shown in HaswelPs figure of a very late blastula, the roof of the blastocoele becomes very 

 thin except in its anterior third. At the posterior end of the segmentation cavity there is 

 a collection of cells of irregular shape. Most of these cells have evidently come from the 

 roof of the blastula ; but Haswell states that some of them are evidently being formed from 

 the parablast of the floor of the cavity, and that this accumulation of cells constitutes the 

 starting point in the formation of the parablast entoderm {end') in Text-figures 53 and 54). 

 In the embryo represented in Text-figure 53, the formation of parablast entoderm is 

 particularly active at the posterior end. 



eet 



Text-figure 54. 

 Sagittal section of a blastoderm of Heterodontus phillipi in an early gastrula stage, 

 ant, anterior; ect, ectoderm; end, entoderm; end', parablast (periblast) entoderm; ent, archenteric cavity. 



After Haswell, 1898, Fig. 7, pi. V. 



