FORMATION OF GASTRULA. 



213 



four cxoderm-cells makes its way into the middle of the 

 cell-mass, and, together with its three fellows, forms a pyra- 

 mid (or tetrahedron). The four exoderm-cells arrange 

 themselves in the form of a cap over the point of this 

 pyramid (Plate II. Fig. 15). This is the beginning of a 

 germinal process which must be regarded as a shortened 

 and vitiated repetition of the inversion of the germ-mem- 

 brane vesicle, and which results in the formation of a Gas- 

 trula. From this time the further cleavage of the manv 

 malian egg adheres to a rhythm which is most essentially 

 similar to that of the Frog's egg. While in the original 

 for primordial) egg-cleavage, the rhythm advances in regular 

 geometrical progressipn (2, 4, 8, 16, 32, 64, 128, and so on); 

 in the modified progression of the mammalian egg, the 

 sequence of numbers is the same as that of the amphibian 

 egg : 2, 4, 8, 12, 16, 24, 32, 48, 64, 96, 160, etc. (Cf. Table V.) 



FIG. 41. Gastrnla of a 

 Mammal (Amphigastrula, of a 

 Rabbit), in longitudinal -section 

 through the axis : e, exoderm- 

 oells (64 brighter and smaller) ; 

 t, entoderm-cells (32 darker and 

 larger) ; d, central entoderm- 

 cells, filling up the primitive in- 

 testinal cavity ; o, external ento- 

 derm-cells, plugging the primi- 

 tive mouth-opening (yelk-plug 

 in the " anus of Rusconi ") 



This depends on the fact that from this time the more 

 vigorous exoderm-cells increase at a quicker rate than the 

 more inert entoderm-cells. The latter always remain 

 behind the former, and are overgrown by them. This pro- 



