217 



these levels being shown at V, 2', 3'. The depressions such as that 

 shown in Pig. 8 c, doubtless appear in surface views as short rough 

 lines, or as areas formed by the confluence of originally isolated 

 spots (compare middle part of lip in the surface sketch, Fig. 4). 

 Such depressions, I infer, are produced by the further separation of 

 the cells which bound the above mentioned gaps. At a slightly later 

 stage, the blastopore appears as a line in surface views, and in 

 sections as a slit-like and 

 pigmented furrow, shown 

 in the sectional view. 

 Fig. 9. The latter figure 

 also shows the develop- 

 ment of pigment in the 

 contiguous surface of 

 yolk cells, the pigment 

 lines (planes) extending 

 forwards from the bottom 

 of the blastopore furrow. 

 It is along these pig- 

 mented surfaces that the 

 yolk cells separate, ac- 

 cording to MOQUIN- 

 Tandon (76), HoussAY 

 ('90), Robinson and 

 AssHETON ('91), giving 

 rise to the slit -like 

 archenteron. That at any 

 rate the extreme anterior 

 part of the archenteron 

 is formed in this way, as 

 a cleavage space in the 

 midst of yolk cells, is 



rendered certain from a study of sections alone. Actual measure- 

 ments made in longitudinal median sections through successive 

 stages, show that the „tongue" of uncleft yolk, lying in front of the 

 archenteron, becomes shorter — that is, the length a—h in such 

 sections as those represented in Figs. 9, 6, 7, lessens with develop- 

 ment. The relative lengths of a—b in these three sections are re- 

 presented by the lines 9, 6, 7 in Fig. 9, all drawn to the same scale. 

 The same inference, as to the gradual shortening of the "tongue" of 

 yolk, is to be drawn from the very accurate figures in Schultze's 



Fig. 9. Part of a median longitudinal section 

 through an early gastrula. Camera Zeiss A 5. d. I. 

 dorsal lip. a — b. "tongue" of uncleft yolk lying in 

 front of the archenteron, which is just beginning to 

 appear. 



