BLASTULA AND GASTRULA 47 



>f Volvox or Sphserozoum (compare Figs. 11 and 12), a point to 

 ,vhich we shall have occasion to return in a later chapter. 



In the blastula of Amphioxus the cells are still all very much 

 ilike, except that those at one pole of the sphere are somewhat 

 arger than the others. Differentiation, however, now sets in in 

 i very marked manner, and the cells thereby become divided into 

 wo distinct groups. That portion of the wall of the hollow 

 >lastula which is formed by the larger cells becomes pushed 

 nwards or invaginated (Fig. 13, VIII), much as a tennis ball 

 may be pushed in by the pressure of the thumb, until it comes 

 nto contact with the inner surface of the remainder of the wall. 

 In this way the original cavity (blastocoel) is obliterated and 

 he embryo takes on the form of a double cup (Fig. 13, IX, X). 

 The cavity of this cup is an entirely new formation. It is the 

 Drimitive digestive cavity of the animal and is known as the 

 nteron or gastral cavity. Its mouth gradually contracts to a 

 narrow aperture, the blastopore. The outer layer of cells forming 

 he wall of the cup is termed the epiblast and the inner the 

 hypoblast, and the two are continuous with one another all round 

 he blastopore. The stage now reached is spoken of as the 

 gastrulajtage. nuu^v^ 



In nearly all Metazoa t/ the blastula stage of development is 

 ollowed by one exhibiting the ftgapmtfal features nf tV>A-ga.Rirn1a. ) 

 or at any rate some indication thereof. The primary differentia- 

 ion of the component cells of the body into an outer epiblast 

 which becomes the ectoderm of the adult) and an inner 

 lypoblast (which becomes the endoderm of the adult), the one 

 serving for protection and for the maintenance of all the 

 necessary relations with the external environment, and the other 

 surrounding a gastral cavity and concerned with the digestion of 

 jhe solid food which the animal captures, is closely correlated 

 with the characteristic animal or holozoic method of nutrition. 



In later stages of development, in all animals higher than the 



IX, young gastrula in longitudinal section ; X, older gastrula in longitudinal section 

 XI, XII, XIII, transverse sections of older" embryos, showing the formation of the 

 coelomic pouches, notochord and neural tube ; XI V, longitudinal section of embryo 

 of about the same age as XII; XV, side view of embryo of same age as .XTFwith 

 the epiblast stripped off from one side to show the mesoblastic somites formed from 

 the coelomic pouches. 



bp. blastopore ; blc. blastocoel or segmentation cavity; c.p. coelomic pouch; ent, enteron ; 

 ep. epiblast ; hyp. hypoblast ; m.s. mesoblastic somites ; not. notochord ; n.p. neuro- 

 pore or anterior opening of neural tube; n.pl. neural plate; n.t. neural tube (central 

 nervous system, formed by folding of neural plate) ; o.c.p. openings of coelomic 

 pouches into enteron. 



