ORIGIN OF THE UKRMINAL LAYERS. 



21 



ms 



Fig. 200. Transverse section 



THROUGH the OVUM OF EuAXES DURING 

 AN EARLY STAGE OF DEVELOPMENT, TO 

 SHEW THE NATURE OF EPIBOLIC INVAGINA- 

 TION. (After Kowalevsky.) 



ej). epiblast; ms. mesoblastic band ; 

 liy. hypoblast. 



Invagination in one form or other occurs in some or all the 



memhers of the following groups : 

 The Dicyemidse, Calcispongise 



(after the amphiblastula stage) 



and Silicispongiae, Coelenterata, 



Turbellaria, Nemertea, Rotifera, 



MoUusca, Polyzoa, Brachiopoda, 



Chaetopoda. Discophora, Gephyrea, 



Chsetognatha, Nematelminthes, 



Crustacea, Echinodermata, and 



Chordata. 



The gastrula of the Crustacea 



is peculiar, as is also that of many 



of the Chordata (Reptilia, Aves, 

 ■ Mammalia), but there is every 



reason to suppose that the gastrulas 



of these groups are simply raodi- 

 , fications of the normal type. 



2. Delamination. Three types of delamination may be dis- 

 tinguished : 



a. Delamination where the cells of a solid morula become 

 '; 1 divided into a superficial 

 epiblast, and a central solid 

 mass in which the digestive 

 cavity is subsequently hol- 

 lowed out (fig. 201). 



h. Delamination where 

 the segmented ovum has the 

 form of a blastosphere, the 

 cells of which give rise by 

 budding to scattered cells in 

 the interior of the vesicle, 

 which, though they may at 

 first form a solid mass, finally 

 arrange themselves in the 

 form of adefinite layer around 

 a central digestive cavity 

 (fig. 202). 



c. Delamination where 

 the segmented ovum has the 

 form of a blastosphere in the 

 cells of which the protoplasm 

 is differentiated into an in- 

 ner and an outer part. By 

 a subsequent process the in- 

 ner parts of the cells become 

 separated from the outer, and 



Fig. 201. Two stages in the development 

 OF Stephanomia pictum, to illustrate the for- 

 mation OF the layers by delamination. (After 

 Metschnikoff.) 



A. Stage after the delamination; e'p. epi- 

 blastic invagination to form pneumatocyst. 



B. Later stage after the formation of the 

 gastric cavity in the solid hypoblast, 'po. poly- 

 pite; t. tentacle; pjj. pneumatocyst; ei^. epi- 

 blast of pneumatocyst; hy. hypoblast surround- 

 ing pneumatocyst. » 



