500 ARTHUR ROBINSON AND RICHARD ASSHETON. 



is now seen from without, c. Ventral lip of blastopore. CH. Notochord. 

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

 rig. 2. E. Space between epiblast and mesoblast. EP. Epiblast. EP' . Epi- 

 dermic layer of epiblast. EP". Nervous layer of epiblast. H. Slit between 

 hypoblast and mesoblast. HY. Hypoblast. HT'. Primitive streak of hypo- 

 blast. J. Line of fusion of the folded-up edges of the neural plate. ME. 

 Mesoblast. 31 E'. Epiblastic or outer layer of mesoblast of primitive streak. 

 ME". Hypoblastic or inner layer of mesoblast of primitive streak. N C. 

 Cranial nerve. NG. Neural groove. N P. Neural plate. N S. Roof of 

 spinal cord. NT. Central canal of spinal cord. N U. Neurenteric 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 blastopore. 

 PS". 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 blastopore 

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

 lateral lips of the original blastopore. P. <S'\ The part of the primitive 

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

 of the neurenteric canal. PSc. The dorsal portion of the neurenteric 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. F. Yolk-cells. F 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. L 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. 



Fig. 5. — A sagittal section of an ovum after the disappearance of the seg- 

 mentation cavity, and when the blastopore is partially closed. X 23. 



Fig. 6. — A portion of a transverse section of an ovum after the separation 



