213 



deepens and spreads, finally nearly filling the cell, in which a com- 

 paratively light margin is left. In Fig. 2 are shown three sketches 

 of a cell, which measures 100 ^i in its long diameter, and occupies 

 a similar position to cell c in Fig 1. The sketches were made at 

 intervals of a few minutes. The chromatophore was originally round ; 

 in Fig. 2 a, it is elongated; in 2 b, it has divided, and faint pigment 

 lines outline the spindle; in 2 c, the division is complete. Mean- 

 time the cell has changed its shape, and it soon divides. 



Appearance of blastopore lip. At the end of the de- 

 lamination period, the cells of the randzone are for the most part 

 40 /< in diameter, while the neighboring yolk cells measure 80 f.t. 

 The zone, however, at its periphery, continues to pass insensibly into 

 the yolk. The first step, visible in surface views, in the formation 

 of the dorsal, and indeed of the rest of the blastopore lip, consists 

 in what I may speak of as the "lining up" of the ectoderm cells. 

 Hitherto there has been absolutely no line between yolk and future 

 ectoderm. Now we find a few of the marginal pale brown cells, at 

 first only two or three (as in Fig. 3), drawn up so that their lower 

 edges (edges towards yolk pole) form a continuous, though by no 

 means even line. The yolk cells on the opposite side of the line 



Fig. 3 



Fig. 4. 



Fig. 3. Upper surface of compressed egg, showing first appearance of dorsal lip 

 of blastopore. 



Fig. 4. Upper surface of the egg shown iu Fig. 3 — a later stage in development 

 of the dorsal lip. 



are about twice the width of the ectoderm cells — in Fig. 3, the 

 yolk cell (tj. c), has a width of 80 /ii, while the ectoderm cells op- 

 posite have a width of 40 /< with a greater length. On each side of 

 this very short lip, the ectoderm and yolk cells pass into one another 

 without a break. Pigment spots simultaneously appear along the 



