No. I.] GAR-PIKE AND STURGEON. 21 



with the yolk ; at the animal pole it consists of about five tiers 

 of cells, and of twice as many at its periphery (PI. I, Fig. 30). At 

 the posterior region of the blastoderm occurs the sharp surface 

 indentation which marks the dorsal lip of the blastopore ; there 

 is as yet, however, no fissure beneath its margin ; the floor of 

 the segmentation cavity has undergone important changes ; it 

 is now formed of a (single) layer of round, many-sized cells, 

 which the writer has seen in cases budded off from the under- 

 lying yolk, but which he believes are mainly derived from the 

 peripheral region, where cell division is now most active ; the 

 line demarking the yolk from this layer is at nearly every point 

 most sharply drawn ; below it, however, no prominent yolk 

 nuclei are to be seen as in the former stage, but the entire 

 upper region of the yolk seems to have acquired a quite differ- 

 ent character ; it now consists of elements which suggest those 

 of the white yolk of the chick. 



A slightly later gastrula is figured in PI. I, Fig. 14, a stage 

 that is prominent on account of the indentation which the 

 blastoderm shows in its hindermost margin ; it is here that the 

 rim of the blastoderm is thickest and most sharply separate 

 from the yolk ; its thickening is not widely different from the 

 shield-shaped embryonic mass of Teleost, although wider and 

 less evident. Sections show but slight changes from the 

 earlier conditions ; in the margin of the blastoderm the cells 

 have become more numerous ; the anterior lip is still closely 

 in contact with the yolk, the posterior is free for a short 

 distance under its immediate margin ; the embryonic thickening 

 near its margin is due almost entirely to an increase in the 

 number of cells of the blastoderm in what has now become 

 the axial region of the embyro. 



In PI. I, Figs. 15 and 16, are figured two later gastrulas : 

 the yolk is in these only to be seen through the rapidly con- 

 stricting blastopore, which in its latest stages is usually cir- 

 cular.i In the first the blastopore is already circular, all traces 

 of the marginal indentation of Fig. 14 having disappeared ; 

 this was observed to take place not by the concrescence of the 



1 The writer was first led to suppose that the circular type of blastopore was 

 abnormal ; this he now believes is the usual form at this stage. 



