288 



THE EVOLUTION OF MAN. 



of dark, multilateral eiitoderm-cells (i), and covered by a 

 light-coloured globular membrane, which is composed of a 

 single stratum of exoderm-cells (e). A ti-ansparent bright- 

 coloured liquid now collects at a point between the two 

 germ-layers ; and this increases so greatly that it expands 

 the exoderm cellular membrane into a large globular vesicle. 

 The mass of entoderm-cells, forming a ball of smaller 

 diameter, remains attached to one ]K)int of the exoderm ; 

 (according to Van Beneden, this point is that of the yelk- 

 plug, o). The entoderm mass now becomes flattened, first 

 assuming a hemispherical, then a lentil-shaped, and finally 

 a discoidal form : this is accomplished by a movement 

 among the cells, which spread themselves out in a one- 

 layered circular disc. 



Fig. 71. — Gastrula of a Mam- 

 mal {Ain2yliigastrula of a Kabbit) 

 iu longitudinal section through 

 the axis : e, exoderm-cells (64, 

 lighter-coloured and smaller) ; i, 

 entoderm-cells (32, darker and 

 larger) ; d, central entoderm-cells, 

 occupying the primitive intes- 

 tinal cavity ; o, jDeripherio euto- 

 derm-cells, plugging the primi- 

 tive mouth-opening (the yelk- 

 plug in the " anus of Euscoui"). 



This vesicular condition of the mammalian germ was 

 detected two centuries ago (1677) by Regner de Graaf He 

 discovered small, globular, transparent vesicles, a\ ith a 

 double membrane, lying free in the uterus of a Rabbit four 

 days after impregnation. But Graaf's statement was not 

 accepted. • It was not till 1827 that these vesicles were 



