2l8 FORMATION OF THE LAYERS. 



The history of the stages immediately following, from about 

 the commencement of the fifth day to the seventh day, when a 

 primitive streak makes its appearance, is imperfectly understood, 

 and has been interpreted very differently by Van Beneden 

 (No. 171) on the one hand and by Kolliker (184), Rauber (187) 

 and Lieberkuhn (18G) on the other. I have myself in conjunc- 

 tion with my pupil, Mr Heape, also conducted some investiga- 

 tions on these stages, which have unfortunately not as yet led 

 me to a completely satisfactory reconciliation of the opposing 

 views. 



Van Beneden states that about five days after impregnation the hypo- 

 blast cells in the embryonic area become divided into two distinct strata, 

 an upper stratum of small cells adjoining the epiblast and a lower stratum 

 of flattened cells which form the true hypoblast. At the edge of the em- 

 bryonic area the hypoblast is continuous with a peripheral ring of the 

 amoeboid cells of the earlier stage, which now form, except at the edge of 

 the ring, a continuous layer of flattened cells in contact with the epiblast. 

 During the sixth day the flattened epiblast cells are believed by Van 

 Beneden to become columnar. The embryonic area gradually extends 

 itself, and as it does so becomes oval. A central lighter portion next 

 becomes apparent, which gradually spreads, till eventually the darker part 

 of the embryonic area forms a crescent at the posterior part of the now 

 somewhat pyriform embryonic area. The lighter part is formed of columnar 

 epiblast and hypoblast only, while in the darker area a layer of the meso- 

 blast, derived from the intermediate layer of the fifth day, is also found. 

 In this darker area the primitive streak originates early on the seventh 

 day. 



Kolliker, following the lines originally laid down by Rauber, has arrived 

 at very different results. He starts from the three-layered condition described 

 by Van Beneden for the fifth day, but does not give any investigations of 

 his own as to the origin of the middle layer. He holds the outer layer to be 

 a provisional layer of protective cells, forming part of the wall of the original 

 vesicle, the middle layer he regards as the true epiblast and the inner layer 

 as the hypoblast. 



During the sixth day he finds that the cells of the outer layer gradually 

 cease to form a continuous layer and finally disappear ; while the cells of 

 the middle layer become columnar, and form the columnar epiblast present in 

 the embryonic area at the end of the sixth day. The mesoblast first takes 

 its origin in the region and on the formation of the primitive streak. 



The investigations of Heape and myself do not extend to the first form- 

 ation of the intermediate layer found on the fifth day. We find on the 

 sixth day in germinal vesicles of about 2-2 2-5 millimetres in diameter 

 with embryonic areas of about "8 mm. that the embryonic area (fig. 136) is 

 throughout composed of 



