102 DEVELOPMENT OF HETERODONTUS (CESTRACION) PHILLIP I, 



but, unless the constituent cells are united by long processes, 

 cannot be said to form a continuous layer. There is every 

 appearance that at this stage there is an active formation of para- 

 blast endoderm cells from the iine-grained parablast below the 

 embryonic rim ; and its substance soon becomes completely 

 divided up into cells. Further forward a similar process goes on, 

 though less actively. A change at the same time takes place in 

 the form and arrangement of the cells of the blastoderm. In 

 the neighbourhood of the embryonic rim they become vertically 

 elongated, their arrangement approximating more and more to 

 thit of the cells of a columnar epithelium, while behind they 

 remain more irregular in shape, and form a stratum several 

 cells thick. It is of importance to observe that, whereas previous 

 to the stage now reached all the cells of the blastoderm were 

 filled with yolk-granules of the smaller size, the vertically- 

 elongated cells now contain yolk only in their lower portions. 



Balfour states that in Pristiurus at his stage B, i.e., at the 

 stage in which the involution to form the endoderm has just 

 begun, the segmentation-cavity has completely disappeared, having 

 become filled with an irregular network of cells. 



The inflection of the blastoderm at the embryonic rim leads to 

 the formation of a fold, the upper layer of which is ectoderm, the 

 lower embryonic endoderm. The latter grows backwards along the 

 entire posterior border of the blastoderm, but more rapidly along 

 the middle line, the cavity below it giving rise to the archenteron 

 (Fig. 7). As it extends backwards it apparently receives con- 

 tributions of new cells from two sources; the greater number 

 of the added cells are derived from the yolkendoderm, but others 

 are derived from the thin layer of cells which has Vjeen described 

 above as lying below the roof of the segmentation-cavity. From 

 the first the endoderm as it becomes formed assumes the character 

 of an epithelium of vertically elongated cells. 



The cavity below the endoderm (archenteron, gastrula-cavity) 

 (Figs. 7 and 8, ent.) is a wide space which is bounded below only 

 by the large-grained yolk with its protoplasmic network. Soon, 

 however, its walls begin to curve inwards anteriorly, and 



