FORMATION AND FATE OF THE PRIMITIVE STREAK. 125 



lip. b. Dorsal lip of blastopore. B C. Blastoporic canal. B L. Blastopore. 

 B L'. Portion of original blastopore not yet closed. B R. Original position of 

 fused layers, that is to say, lips of blastopore at the time of the first formation 

 of the circular blastopore. B B'. Position of fused layers or lips of blastopore 

 after concrescence to a greater or less extent of the lateral lips of the original 

 blastopore. B B". The dorsal portion of the lateral lips of the blastopore 

 folded over with the neural plate, so that which was their ventral or inner 

 surface is now seen from without, c. Ventral lip of blastopore. C H. Noto- 

 chord. D. Diverticulum from archenteron towards mesoblast. d. Line of 

 section of Fig. 2. E. Space between epiblast and mesoblast. E P. Epiblast. 

 E P'. Epidermic layer of epiblast. E P". Nervous layer of epiblast. H. Slit 

 between hypoblast and mesoblast. If T. Hypoblast. H Y'. Primitive 

 streak of hypoblast. </. Line of fusion of the folded-up edges of the neural 

 plate. ME. Mesoblast. ME'. Epiblastic or outer layer of mesoblast 

 of primitive streak. M E". Hypoblastic or inner layer of mesoblast of primi- 

 tive streak. N C. Cranial nerve. N G. Neural groove. NP. Neural plate. 

 N S. Koof of spinal cord. N T. Central canal of spinal cord. N U. Neuren- 

 teric canal. P G. Primitive groove. P S. Primitive streak. P S'. The part 

 of the primitive streak ventral to the anus, i.e. ventral lip of original blasto- 

 pore, P S". The part of the primitive streak which is continuous with the 

 posterior end of the notochord, i.e. dorsal lip of original blastopore. P S."' 

 The part of the primitive streak which lies between that portion of the blasto- 

 pore which remains open longest and the anus, i.e., the greater part of the 

 fused lateral lips of the original blastopore. P S iv . The part of the primitive 

 streak which is folded up with the neural folds and forms the posterior wall of 

 the neur enteric canal. P S c. The dorsal portion of the neur enteric canal. 

 S. Line of separation of mesoblast from chorda. S G. Segmentation cavity. 

 x. The lowest extremity of that part of the archenteron which is bounded 

 posteriorly by the fused lips of the blastopore. Y. Yolk cells. Y P. Yolk- 

 plug. Z, Z', ZZ, ZZ'. Furthest points from the edge of the blastopore, at 

 which a distinct fusion of germinal layers is perceptible. * *. Two points of 

 epiblast beyond the neural plate, which meet and fuse when the folding up of 

 the neural plate is completed, f f. Two points of the epiblast beyond the 

 dorsal portion of the primitive streak, which meet and fuse when that portion 

 of the primitive streak is folded up with the neural plate. 



Fig. 1. — Sagittal section of a portion of the ovum represented in Fig 4, 

 showing the dorsal lip of the blastopore and the area of fusion in front of it. 

 The cavity of the archenteron, and the pigmented streak in front of it which 

 indicates the line of extension, x 50. 



Fig. 2. — A transverse section through the pigmented area in front of the 

 archenteric cavity. The section is taken in the direction of the line d in Fig. 

 1. x 115. 



Fig. 3. — A sagittal section through the posterior lip of the blastopore of the 

 ovum represented in Fig. 4. x 115. 



Fig. 4.^A sagittal section of an ovum at the period of completion of the 

 blastopore, x 23. 



