ORIGIN OF T1IK GERMINAL LAYERS. 



'Ill 



200. TEANSVERSB SECTION 

 THE OVDM OF EUAXES DUEING 

 AN EAKLY STAGE OF DEVELOPMENT, TO 

 SHEW THE NATURE OF EPIBOLIC INVAG1NA- 

 TION. (After Kowalevsky.) 



ep. epiblast; ins. mesoblastic band ; 

 liy. hypoblast. 



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

 members of the following groups : 



The Dicyemidse, Calcispongiae 

 (after the amphiblastula stage) 

 a.nd Silicispongiae, Coelenterata, 

 Turbellaria, Nemertea, Rotifera, 

 Mollusca, Polyzoa, Brachiopoda, 

 Chsetopoda, Discophora, Gephyrea, 

 Chsetognatha, Nematelminth.es, 

 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 gastrulae 

 of these groups are simply modi- 

 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 

 divided into a superficial 



epiblast, and a central solid 

 mass in which the digestive 

 cavity is subsequently hol- 

 lowed out (fig. 201). 



b. 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 



FlG. 201. TWO STAGES IN THE DEVELOPMENT 

 OF STEPHANOMIA PICTUM, TO ILLUSTRATE THE FOR- 

 MATION OF THE LAYERS BY DELAMINATION. (After 



Metschuikoff.) 



A. Stage after the delamination; ep. epi- 

 blastic invaginatiou to form pneumatocyst. 



B. Later stage after the formation of the 

 gastric cavity in the solid hypoblast. po. P1.V- 

 pite; t. tentacle; pp. pneuniatocyst ; ep. epi- 

 blast of pneiimatocyst ; 1<y. hypoblast surround- 

 ing pneurnatocyst. 



