GENERAL EMBK YOLOG Y. 



I8 7 



the equal and the unequal cleavage can be found in all the 

 groups of the metazoa. 



Blastula. Even during the first stages of segmentation 

 there is formed a cavity, the cleavage cavity, in the interior 

 of the egg between the cells ; with the progress of devel- 

 opment this cavity becomes continually larger. Around it 

 the cells lie in the form of a one-layered or of a many- 

 luyered epithelium and form the blastoderm. Hence the 

 name for this stage, vesicula blastodennica or, briefly, bias- 

 tula. The more yolk there is present, the smaller is the 

 diameter of the cleavage cavity ; in centrolecithal eggs with 

 superficial cleavage it is entirely absent. 



4. Formation of tJic Germ-layers. 



Gastrula. Besides the blastula there is still a second 

 stage of development, the gastrula or the two-layered em- 

 bryo, which is common to all the metazoa. 

 This stage is understood easiest in the 

 case of eggs which have an equal cleavage 

 (Fig. 101, E] ; here it has the form of a 

 double-walled cup with a wider or nar- 

 rower mouth. The cavity of the cup (the 

 primitive digestive tract orarckenterori) is 

 the rudiment of the most important part 

 of the digestive system ; the opening is 

 the primitive mouth or the prostoma 

 (called also the blastopore). Of the two 

 layers of cells forming the wall of the cup 

 and uniting at the prostoma, the exter- FlG IOI _ Gastrulation 

 nal is the ectoblast or outer gcrm-laver, 

 the internal the cntoblast or inner gerui- 

 laycr. In the gastrula stage, we meet 

 for the first time the formation of germ- 

 layers, i.e., the formation of definite 

 embryonic layers marked off from each 

 other, the cells not yet differentiated, 



Atnpliio \us (After 

 Hatschek.l Tilt- animal 

 pcilr here is above, and 

 the vegetal pole htlnw, 

 in comparison with l-'ii;. 

 93. In Fiji. A the cells 

 of the vegetal pole are 

 beginning to sink in />, 

 the invagination com- 

 pleted ; the cleavage cav- 

 ity reduced to a slit be- 

 tween the entoblast (fit) 

 and the ectoblast (ek) ; a, 

 gastru'a-mouth. 



from which organs arise 

 tological differentiation. 



through 



organological 



and his- 



