BY WILLIAM A. HASWELL. 105 



eventually meet below so as completely to enclose the archenteric 

 canal in its anterior portion — the enclosure gradually extending, 

 backwards. 



EXPLANATION OF FIGURES. 



a7if., anterior end of blastoderm. 



ect., ectoderm. 



ect.^, superficial layer of cells distinguishable before the comp'e- 



tion of segmentation. 

 end., endoderm. 

 end.'^, parablast endoderm. 

 ent. , archenteron. 

 para., bed of fine-grained yolk with parablast nuclei. 



Fig. 1. — Sagittal section of the blastoderm of Heterodontus Phillipi at a 

 late stage of segmentation, showing the beginnings of the seg- 

 mentation-cavity anil the superficial layer. 



-Similar section of a somewhat later stage. 



-Stage with well-defined segmentation-cavity at the posterior end. 



-Somewhat later stage, in which tlie segmentation-cavity has become 

 extended forwards an<l in which the first indication of the 

 invagination is to be distinguished. 



-Later stage in which the involution has begun, and the rudiments 

 of the parablast endoderm have become formed. 



-Stage in which the blastoderm has become considerably extended 

 forwards and the parablast endoderm has become developed. 



-vSimilar section of a blastoderm in which the archenteron has 

 become well established. 



-Transverse section of a blastoderm of a somewhat later stage, but 

 before the first appearance of the notochord. 



