70 CHIM^ROID FISHES AND THEIR DEVELOPMENT. 



The fourth stage in gastrulation appears in surface view in plate v, fig. 32, and 

 is but two days older than the preceding, plate v, fig. 31. It shows the following 

 advances: (i) The embryo proper makes its appearance in a small depression, and 

 is conspicuous on account of the dark adjacent shadows; (2) the center of the 

 blastoderm rises as a circular plateau, leaving at its base a well-marked flattened 

 rim; surrounding this the circumgerminal ring has largely faded away, its nuclei 



pm 



+ 



Fig. 65. Sagittal section of gastrula slightly older than the preceding, 

 ft, Archenteron. sc t Segmentation cavity. 

 pntt Posterior mass of cells (in region of ventral lip of blastopore). 



Fig. 65 A. Section parallel to preceding, but situated further at the side. 

 Fig. 65 B. Section parallel to preceding, but more nearly marginal. 

 Fig. 65 C. Detail showing coarse yolk. 



now having passed far out over and into the yolk;* (3) anteriorly the segmentation 

 cavity is becoming restricted to a small area, appearing in surface view as a light- 

 colored tract near the rim of the blastoderm. Two sections of this blastoderm are 

 figured, one sagittal (fig. 66), the other (fig. 66 A) passing between the points A A 

 shown in surface view. Contrasting the sagittal section (fig. 66) with that of the 



* The figure represents the circumgerminal zone as too wide and conspicuous, an inaccuracy which was noticed 

 too late for correction. 



