y38 Bashford Dean Memorial Volume 



The gastrula of Heterodontus (presumably japonicus), represented by serial sagittal 

 sections found among Dean's embryological preparations, is in a stage considerably later 

 than HaswelPs embryo shown in median sagittal section in my Text-figure 54. A median 

 sagittal section of Dean's gastrula is represented in Text-figure 56. The section cuts 

 through the entire length of the blastoderm, which measures 8 mm. (on the slide); but the 

 embryo proper, shown in the figure, is only about one millimeter long. Both ectoderm and 

 the definitive entoderm, as shown in the drawing, are decidedly thick; but at the anterior 

 end of the embryo the ectoderm gradually decreases in thickness until in the extra- 

 embryonic portion of the blastoderm it is a simple squamous epithelium. The periblast 

 is represented by a pale 2;one of predominantly fine yolk granules interspersed with 

 cytoplasm, underlying the archenteric cavity and extending a short distance anterior to it. 

 Beneath the archenteric cavity it is much thicker (deeper) than it is anteriorly. This 

 thick portion lacks nuclei, but contains a number of fairly large vacuoles. Underlying the 

 anterior end of the definitive entoderm, and extending forward beneath the ectoderm, 

 is a thin layer of irregularly shaped cells that constitute the periblast entoderm. They 

 seem to grade into the definitive entoderm, and perhaps contribute to it ; anteriorly, the 

 layer becomes even thinner and in the extra-embryonic portion of the blastoderm it 

 is represented (in sections) by a single row of sparsely distributed cells lying between the 

 ectoderm and the yolk mass. Underlying the thick portion of the periblast entoderm, 

 a little distance anterior to the archenteric cavity, there are a few periblast cells (not 

 merely nuclei) imbedded in the yolk. 



The advanced gastrula of Torpedo figured in median sagittal section by Ziegler (1892) 

 is in about the same stage as the embryo of Heterodontus japonicus represented in my 

 Text-figure 56. Ziegler's figure shows the section continuing throughout the entire 

 length of the blastoderm. A striking difference is the much greater thickness (2 to 5 

 cells deep) and compactness of the periblast entoderm in the extra-embryonic portion of 

 the blastoderm, as compared with Dean's gastrula in which this layer is only one cell 

 thick and the cells are separated by fairly wide intercellular spaces. 



THE YOLK BLASTOPORE 



Before going further with a description of strictly embryonic development it seems 

 advisable to give brief attention to the formation and closure of the yolk blastopore, which 

 may be regarded as a delayed phase of gastrula tion. 



The yolk blastopore is simply that portion of the surface of the yolk mass which, 

 subsequent to the beginning of gastrulation, is not yet covered by the blastoderm (Figures 

 46 to 59, plates IV and V). It is bounded by the rim of the blastoderm and is continuous 

 with the floor of the archenteric cavity. The name blastopore seems more appropriate 

 after the blastoderm has covered more than half the yolk mass, but the morphological re- 

 lations are the same in earlier stages. One reason for regarding the yolk blastopore as 

 related to gastrulation is that, in heavily yolk-laden eggs, circumcrescence (overgrowth of 

 the yolk by the blastoderm) is an important factor in gastrulation — it assists in laying 



