THE PRIMITIVE STREAK. 71 



anus through the primitive streak. The somites of this embryo 

 differ from those of the earlier stages only in the fact that 

 they coutaiu a well-marked cavity. Their walls are still in 

 continuous contact with the ectoderm and endoderm respectively. 

 The primitive streak of this stage needs, however, more con- 

 sideration. In the first place the primitive groove has become 

 much more marked and extends over a greater distance than in 

 the earlier stages (Plate II, fig. 25, and Plate VI, fig. 5 d — /). 

 Immediately behind the blastopore the primitive streak con- 

 sists of but few nuclei, which form a layer placed between the 

 ectoderm and endoderm, and extending laterally as far as the 

 mesoblastic bands. The latter structures here bend in towards 

 the middle line, so that a few sections behind (fig. 5 e) they 

 have reached and become indistinguishable from the primitive 

 streak. The primitive streak in this region is much more 

 bulky and consists of a large mass of nuclei. It extends 

 back for about thirty sections behind the blastopore. There 

 can be no doubt that the mesoderm of the developing hind end 

 of the embryo, which now begins to grow forward so as to lie 

 with its ventral side on the ventral side of the anterior part (vide 

 Plate II, figs. 26 — 31), is derived from the continually proliferat- 

 ing cells of the primitive streak. The latter is indeed simply 

 the growing point of the embryo, and in it the three layers of 

 the embryo are united and indistinguishable from one another. 

 It persists until the full complement of somites is obtained and 

 then disappears. Some time before its disappearance it be- 

 comes relatively of less extent, and the anus consequently 

 comes to lie nearer the hind end of the body until, at its com- 

 plete disappearance, the anus has gained its permanent position 

 at the hind end of the body. Plate VI, fig. 6 d, illustrates 

 the structure and appearance of the streak during Stage c. 

 The section was taken at the hind end and cuts the embryo, 

 the ventral flexure of which is beginning, at two points, viz. 

 in the region of the primitive streak and in the region of the 

 anus. The later stages of the primitive streak are shown best 

 in longitudinal vertical sections (Plate VII, figs. 28 — 30). 

 The large pole-cell of the primitive streak, visible during 



