FORMATION AND PATE OF THE PRIMITIVE STREAK. 127 



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 ediblastic 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. 2\ mm. Tadpole. A horizontal section parallel to the preceeding 

 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. 2£ 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 proctodaeum ; 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. 2J 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 B between the two concentric circles represents the original 

 position of B Bf. The asterisks denote two points in the epiblast which will 

 come together when the neui'al 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 blastopore 

 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 

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

 diagrammatically represents the anus as a re-opening 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 



