THE DEVELOPMENT OF THE MOLE. 141 



derm at the front end of the embryo ; beyond that point no mesoblast is 

 present in the middle line. 



Pig. 12, Stage E. — Transverse section through the cephalic plate of an 

 embryo with three protovertebrse. At the point where this section is taken 

 the flat cephalic plate is completely folded off from the yolk-sac. The narrow 

 fore-gut is shown as a tube {al. c.) immediately below the cephalic plate. A 

 few scattered mesoblast cells extend between the two layers of epiblast. 



Fig. 13, Stage E.- — ^Transverse section through an embryo 1'97 mm. long, 

 with only a single protovertebra. The section is taken in front of the proto- 

 vertebra, and shows the indication of a split of the mesoblast into somatic and 

 splanchnic layers throughout its whole depth. The medullary groove is wide 

 and deep. The notochord is formed of flattened cells. 



Figs. 14 and 15, Stage E. — Tranverse sections through the same embryo 

 which is drawn in Fig. 1. 



Fig. 14 is taken in front of the protovertebra, where the mesoblast is 



split into somatic and splanchnic layers only at the periphery. 

 Fig. 15 passes through a protovertebra. The body cavity extends in- 

 wards as far as the intermediate cell mass (i. c. m.) in the peripheral 

 mesoblast, and a small cavity is also present within the protovertebra. 



Figs. 16, 17, and 18, Stage F. — Transverse sections through embryos with 

 five protovertebrse. 



Pig. 16 is a section through the head. The cephalic plate is grooved in 

 the middle line and at either side where the wide optic grooves are 

 situated. When the external edges meet in the middle line these optic 

 grooves will be converted into vesicles communicating by a wide aper- 

 ture with the central canal. The notochord is not yet separated from 

 the hypoblast. 

 Fig. 17 is through the trunk ; the medullary groove is narrower, and the 

 notochord more defined than in Fig. 15, which is a section through a 

 similar region of an embryo of Stage E. 

 Fig. 18 is a section through the sinus rhomboidalis, and shows the 

 anterior end of the primitive streak and the amnion. 

 Pig. 19, Stage G. — Transverse section through the head of an embryo with 

 eight protovertebrse, 2-49 mm. long. The head at this point is completely 

 folded off, and the medullary groove (still open) will at this point give rise to 

 the mid-brain. A few sections further forward the optic vesicles are cut, 

 projecting outwards from tlie central canal, and it is on account of the 

 proximity of these structures that the wide space between the external 

 epiblast and the walls of the medullary canal is present here. This space is 

 here filled with stellate mesoblast cells. The two grooves in the epiblast on 

 the under surface on either side the middle line converge posteriorly, and 

 where they meet the mouth will eventually be formed. 



