70 



CHIM^ROID FISHES AND THEIR DEVELOPMENT. 



The fourth statue in p;astrulation 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 circumgcrminal ring has largely faded away, its nuclei 



6cB 



65' 



'5Sig|!:i??i, 



^m 



V* 



Fig. 65. — Sagittal section ot gashula slightly older than the preceding. 



", Archenteron. -S'", Segmentation cavity. 



7>'", 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. 



