. Anatomischer Anzeiger. Anatomy, Comparative; Anatomy, Comparative. 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 t
. Anatomischer Anzeiger. Anatomy, Comparative; Anatomy, Comparative. 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 Please note
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