66 ANIMAL PHYSIOLOGY. 
has been formed. This is rather to be considered as a type of 
one form of segmentation than as applicable to all, for even 
at this early stage differences are to be noted in the mode of 
segmentation which characterize effectually certain groups of 
animals; but in all there is segmentation, and that segmenta- 
tion is rhythmical. 
Fie. 69.—The elbeeane of a frog’s eee (10 times enlarged). .A, the parent-cell ; B, the two 
first cleavage-cells ; C, 4 cells; D, 8 cells (4 animal and 4 vegetative) ; E, 12 cells (8 animal 
and 4 vegetative); F, 16 cells (8 animal and 8 vegetative); G, 24 cells (16 animal and 8 
weneeve) | Hy, 3% cells; J, 48 cells; K, 64 cells ; ee 96 cleavage-cells ; M, 160 cleavage- 
cells (128 animal and 82 vegetative). (Haeckel.) 
Segmentation results in the formation of a multicellular 
aggregation which, sooner or later, incloses a central cavity 
(segmentation cavity, blastocele). Usually this cell aggrega- 
tion (blastula, blastosphere) is reduced to a single layer of in- 
vesting cells. 
The Gastrula——Knsuing on the changes just described are 
others, which result in the formation of the gastrula, a form of 
cell aggregation of great interest from its resemblance to the 
Hydra and similar forms, which constitute in themselves inde- 
pendent animals that never pass beyond that stage.. The blas- 
tula becomes flattened at one pole, then depressed, the cells at 
