BLASTULA AND GASTEULA 47 



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

 which we shall have occasion to return in a later chapter. 



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

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

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

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

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

 blastula which is formed by the larger cells becomes pushed 

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

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

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

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

 the 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 

 primitive digestive cavity of the animal and is known as the 

 enteron or gastral cavity. Its mouth gradually contracts to a 

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

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

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

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

 gastrula stage. 



In nearly all Metazoa the blastula stage of development is 

 followed by one exhibiting the essential features of the gastrula, 

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

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

 (which becomes the ectoderm of the adult) and an inner 

 hypoblast (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 

 the 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; XIV, longitudinal section of embryo 

 of about the same age as XII ; XV, side view of embryo of same age as XIV with 

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

 the ccelomic pouches. 



bp. blastopore ; lie. blastoccel or segmentation cavity ; c.p. ccelomic pouch ; ent. enteron ; 

 ep. epiblast; liyp. 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. 



