FORMATION AND PATE OF THE PRIMITIVE STREAK. 503 



Fig. 18 have been brought together so as to almost coincide. This figure also 

 diagrammatically represents the anus as a reopening of the ventral part of 

 the original blastopore. This diagram and the diagram of Fig. 18 represent 

 nearly the stages of the drawings Fig. 23 and Fig. 11 respectively. 



Fig. 20. — 4^ mm. Tadpole. A horizontal section through the posterior end 

 at a level corresponding to that of Fig. 15, but of an older embryo. Instead 

 of the layers being fused along the middle line, they are now separated and 

 distinct. Camera drawing, x 55. 



Fig. 21. — 44 mm. Tadpole. A horizontal section through the posterior end 

 at a level corresponding to that of Fig. 16, that is to say, through the anus, 

 which is now completely formed. Camera drawing, x 55. 



Fig. 22. — 4| mm. Tadpole. A horizontal section through the posterior end 

 at a level corresponding to that of Fig. 17. These three sections. Figs. 20, 

 21, and 22, are from the same series. The sections are not stained, and the 

 drawings have been made to represent the actual shades of the three layers as 

 nearly as possible. Camera drawing, x 55. 



Fig. 23. — 2i mm. Tadpole. A surface view of the posterior end of a 

 tadpole, in which the neural folds have met and are fusing. The "blastopore" 

 is still open, and the anus is conspicuous. The horizontal lines numbered 14, 

 15, 16, and 17 are drawn at the levels at which the sections were taken, from 

 which Figs. 14, 15, 16, and 17 were drawn. The bracket, PS, includes 

 the dorsal and ventral limits of the primitive streak. This figure was drawn 

 partly from a specimen hardened in Kleinenberg's picro-sulphuric acid, and 

 partly from a model constructed by pasting together in order pieces of card- 

 board corresponding to a complete series of camera drawings. 



Fig. 24. — A sagittal section of the posterior three quarters of a frog embryo, 

 in which the neural plate is beginning to fold up, and in which the anus of 

 Rusconi has become much reduced. The yolk-plug has been retracted, 

 and the embryo is lengthening. The extent of the primitive streak is indi- 

 cated by the bracket P S, and by the diagonal shading. 



Fig. 25. — 2i mm. Tadpole. A semi-diagrammatic sagittal section through 

 the posterior half of a frog embryo, of the same age as that from which Fig. 

 23 was drawn. The extent of the primitive streak is indicated by the bracket 

 P/S, and by the diagonal shading. The neural plate has completely folded 

 over, but has not yet separated from the external epiblast. This figure shows 

 the dorsal portion of the primitive streak folded over with the neural plate, 

 and with it about to be separated from the adjoining epiblast. The blasto- 

 pore has not quite closed in the middle line. The neurenteric canal is seen to 

 be formed of two portions; the ventral, being the original opening from arch- 

 enteron to exterior, the blastoporic canal {B.C.) or blastopore, and a dorsal, 

 the canal through the folded-up lateral lips of the blastopore, or 



