180 FORMATION OF THE LAYERS. 



has by no means attained its greatest dimensions, it has nevertheless 

 grown from about 0'09 mm. — the size of the ovum at the close of 

 segmentation — to about 0'28. It is enclosed by a membrane formed 

 from the zona radiata and the mucous layer around it. The blas- 

 todermic vesicle continues to enlarge rapidly, and during the process 

 the liypoblastic mass undergoes important changes. It spreads out on 

 the inner side of the epiblast and at the same time loses its lens-like 

 form and becomes flattened. The central part of it remains however 

 thicker, and is constituted of two rows of cells, while the peripheral 

 part, the outer boundary of which is irregular, is formed of an 

 imperfect layer of amoeboid cells which continually spread further 

 and further within the epiblast. The central thickening of the 

 hypoblast forms an opaque circular spot on the blastoderm, which 

 constitutes the commencement of the embryon^ area. 



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), Kauber (187) and Lieberklihn (186) on 

 the other. I have myself in conjunction with my pupil, Mr Heape, 

 also conducted some investigations on these stages, which have un- 

 fortunately not as yet led me to a completely satisfactory recon- 

 ciliation 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 ar- 

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

 described by Van Beneden for the fifth day, but does not give any investi- 

 gations 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 



I 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 

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



