THE DEVELOPMENT OP THE MOLE. 143 



here. The false amnion is formed of flattened somatic mesoblast and 

 columnar epiblast cells, the latter will eventually fuse with the 

 uterine epithelium. 

 Pig. 29 is also a section through the middle of the embryo. The cells of 

 the protovertebrse are here seen to be somewhat elongated on the right 

 side of the section, while on the left the cavity of the protovertebra is 

 shown partially filled by a cove of mesoblast cells. This was also 

 shown in Fig. 28. The hour-glass shape of the neural canal at this 

 point is also to be observed. 

 Figs. 30 and 31 are from the hinder portion of the trunk of the embryo. 

 The neural canal is not closed, the protovertebrse are not so com- 

 pletely isolated from the neighbouring mesoblast, and the notochord, 

 which is larger than in former sections of this stage, is not at all 

 isolated from the hypoblast in Fig. 31. In both these sections the two 

 dorsal aortse, which were present in all the sections from Fig. 23 to 

 Fig. 29, are here giving off branches to the yolk-sac. The vitelline 

 arteries {vt. a.), and posterior to this point, the aortse themselves, no 

 longer exist. 

 Fig. 32 is a section behind the former sections, and just in front of the 

 primitive streak. The widely open medullary groove is here called the 

 sinus rhomboidalis. 

 Fig. 33 is a section through the primitive streak; the medullary folds are 

 growing round it and will shortly completely enclose its front end. 

 Fig. 34;, Stage H. — A median longitudinal section through the head of an 

 embryo, in which the following points are shown : — The cranial flexure ; the 

 fore-, mid-, and hind- brains; the notochord separated from the hypoblast, 

 except along the front wall of the alimentary canal ; the ventral prolongation 

 from the primitively straight fore- gut {al. c), and the pro-amnion formed of 

 epiblast and hypoblast only {pro. am.). 



Fig. 35, Stage H. — Median longitudinal section through the hind end of an 

 embryo. The dorsal pit (anal pit) and the ventral pit (allantoic pit) separate 

 the anterior from the posterior portions of the primitive streak. 



Figs. 36 — 42, Stage H. — Transverse sections through various regions of an 

 embryo, to show the formation of the notochord. 



Fig. 36 in the anterior region shows a lumen within the notochord. 

 Fig. 37. In the anterior region : notochord is rod like, and is separated 



from the hypoblast. 

 Fig. 38. In the anterior region : notochord is a flattened band-like struc- 

 ture ; it is separated from the hypoblast. 

 Fig. 39. In the middle region : the notochord is not yet separated from 

 the hypoblast in the middle line, although it is so separated at either 

 edge. The lateral ingrowth of the hypoblast is shown. 

 Fie. 40 from the posterior region, shows relations similar to those seen 



