XTII 



PHYLUM CHOKDATA 



131 



dence with this, segmentation is complete but unequal, the morula 

 consisting of an upper hemisphere of small cells or micromeres 

 (Fig. 813, mi. m.), free from yolk, and of a lower hemisphere of 

 large cells or megameres (mg.m.), containing much yolk. In the 

 blastula stage (D) the 

 segmentation-cavity or 

 blastoccele (bid.) is 

 situated nearer to the 

 upper than to the 

 lower pole. A trans- 

 verse semilunar groove 

 appears which is 

 bounded by a promi- 

 nent rim towards the 

 future dorsal and 

 anterior side : this 

 is the blastopore 

 (blp.). The megameres become gradually enclosed by the micro- 

 meres as a result of a process which is partly invagination, 

 partly epiboly. During this process the segmentation-cavity 

 becomes displaced by the archenteron. The dorsal and ventral 



FIG. 812. Petromyzon marinus. The urinogenital sinus 

 with posterior end of intestine and part of left kidney. 

 a. anus ; int. intestine ; k. left kidney ; r. rectum 

 u.g.p. urinogenital papilla ; u.g.s. urinogenital sinus ; 

 ur. left ureter ; x, x', apertures of ureters into urinogeni- 

 tal sinus ; y, bristle passed into right genital pore ; 

 z, bristle passed from urinogenital aperture into sinus. 

 (From Parker's Zootomy.) 



FIG. 813. Petromyzon. A and B, two stages in segmentation ; C, early embryo from the 

 posterior aspect ; I), section of blastula stage ; E, section of gastrula stage, blp. blastopore ; 

 bid. blastocoale or segmentation-cavity ; k. keel ; mg. in. megameres ; mi. m. micromeres. 

 (After Shipley and Kupffer.) 



walls of the latter, unlike those of the archenteron of Amphioxus, 

 differ widely from one another, the ventral wall being composed 

 of a thick mass of yolk-cells (megameres), while the roof is com- 

 paratively thin and consists of two or three layers of rounded 



T 2 



