266 



ELEMENTS OF BIOLOGY 



BLASTOPORE 



hollow sphere. In the simplest cases this is effected by differences in 

 growth that cause the impinging of one hemisphere of the blastula 

 into the other, a process which may be roughly compared to the 

 impressing of one half of a hollow rubber ball into the other half 

 so that the result is a double-walled hemisphere, the inner and outer 

 layers being continuous around a rim (Fig. 179). This process in 

 development is called gastrulation and establishes the two primary 

 layers, the ectoderm and the endoderm, continuous around the lips 

 of the circular opening, the blastopore. The blastocoele is thus 



partially obliterated. The inner, 



ARCHENTERON i-i r 11 t 



sac-like cavity lormed by the 

 endoderm is the rudiment of 

 the alimentary tube, the ar- 

 CHENTERON. Again there are 

 grounds for comparing this 

 stage in the development of 

 higher animals with the adult 

 of the lower forms, for it will 

 be recalled that the ccelenterate, 

 hydra, consists of a two-layered 

 hollow sac. 



Cleavage and gastrulation as described for the simplest cases take 

 place in this fashion only in eggs with a relatively small amount of 

 yolk fairly equally distributed throughout the egg, or isolecithal 

 eggs. In eggs with more yolk unequally distributed the processes 

 are much modified. Such eggs are termed teleolecithal eggs. The 

 amount and distribution of yolk varies among teleolecithal eggs. 

 For the ^^^ organization and cleavage of a teleolecithal ^g^ con- 

 taining a relatively small amount of yolk a study may be made of 

 the early development of the frog's ^gg. Because of their inter- 

 mediate yolk character, such eggs are sometimes known as 

 mesolecithal. 



ECTODERM ENDODERM 



Fig, 179. — The gastrula stage of 

 Amphioxus. 



