68 COMPARATIVE PHYSIOLOGY. 



form of segmentation than as applicable to all, for even at 

 this early stage differences are to be noted in the mode of seg- 

 mentation which characterize effectually certain groups of ani- 

 mals ; but in all there is segmentation, and that segmentation 

 is rhythmical. 



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, blastophere) is reduced to a single layer of invest- 

 ing cells. 



The Gastrula. — Ensuing on the changes just described are 



Fig. 66.— 31astala and gastrula of ampMoxus (Clans, after Hatschek;). A, blastnla 

 with flattened lower pole of larger cells; B, commencing invagination; C, gastru- 

 lation completed; the blastopore is still widely open, and one of the two hinder- 

 pole mesoderm cells is seen at its ventral lip. The cilia of the epiblast cells are 

 not represented. 



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 pafes beyond that stage. The blas^ 

 tula becomes flattened at one pole, then depressed, the cells at 

 this region becoming more columnar (histological differentia- 

 tion). This depression (invagination) deepens until a cavity is 

 formed (as when a hollow rubber ball is thrust in. at one part 

 till it meets the opposite wall), in consequence of which a two- 

 layered embryo results, in which we recognize the primitive 

 mputh (blastopore) and digestive cavity (archenteron), the outer 

 layer (ectoderm) being usually separated from the inner (endo- 

 derm) by the almost obliterated segmentation cavity. Such a 

 form may be provided with cilia, be very actively locomotive, 

 and bear, consequently, the greatest resemblance to the perma- 

 nent forms of some aquatic animals. 



The changes by which the segmented oosperm becomes a 

 gastrula are not always so direct and simple as in the above- 



