602 ARTHUR ROBINSON AND RICHARD ASSHETON. 



pore," and the sections 15, 16, and 17 are the 16th, 32nd, and 44th sections 

 respectively below section 14. 



Fig. 14. A horizontal section through the posterior end of the embryo, 



and therefore a section transverse to the longitudinal axis of the 



primitive streak. The section is taken through the centre of the small 



part of the original blastopore which is still open, which, however, is 



seen to be on the point of closing. The three germinal layers are 



fused at the blastoporic lip, and two layers of the mesoblast may be 



distinguished — a denser outer or epiblastic layer, ME', and a looser 



inner or hypoblastic layer, ME" . The cells of the latter are further 



characterised by being rounder and more deeply pigmented. Drawn 



with camera, x 55. 



Fig. 15. 21 mm. Tadpole. A horizontal section parallel to the preceding 



section, taken through about the centre of the primitive streak. It 



presents practically the same features as the preceding figure, except 



that at this point the blastopore has completely closed, and the opposite 



lips have fused, forming a typical primitive streak. Camera drawing, 



X 55. 



Fig. 16. 2i mm. Tadpole. Horizontal section through the posterior part 



of the embryo. This section passes through that portion of the 



primitive streak in which the anus will shortly appear. The primitive 



groove is at this point very much deeper, forming that which is usually 



called the proctodseum ; but complete perforation has not yet occurred. 



The line of perforation is, however, foreshadowed as it were by a deep 



irregular line of pigment. Camera drawing, X 55. 



Fig. 17. 21 mm. Tadpole. A horizontal section of posterior end of an 



embryo ventral to the anus. This is the thirteenth section behind the 



anus. The primitive groove is still well marked. As there is no true 



hypoblast here, there is fusion only between epiblast and mesoblast. 



Drawn with camera, X 55. 



Fig. 18. — A diagram to show the relation of the neural plate to the 



"blastopore" and rest of the primitive streak. The dotted area represents 



the neural plate. The shaded area B B! represents the primitive streak at 



the time just prior to the commencement of the folding up of the neural 



plate. The space B R between the two concentric circles represents the 



original position of B R' . The asterisks denote two points in the epiblast 



which will come together when the neural plate has folded up, and the daggers 



denote two other points alongside the blastoporic lips which will come together 



as explained in the text. 



Fig. 19. — A diagram to show the relation of the neural plate to the blasto- 

 pore and rest of the primitive streak, and to show the way in which a portion 

 of primitive streak becomes folded up along with the neural folds. It will be 

 seen that here the folding is nearly completed ; the asterisks and daggers of 



