962 



MATURATION OF THE OVUM. 



The germinal vesicle, which as a typical stage in the development 

 of numerous animal species is also designated the blastula, thus consists 

 of a vesicle with walls made up of a single layer of cells, as represented 

 diagrammatically in Fig. 372, i, in sagittal section (from amphioxus). 

 The subsequent important formative process consists in the develop- 

 ment from the blastula of a hollow structure, whose walls consist of a 

 double layer of cells. This process of transformation can be best 

 followed in the ovum of the lancet-fish (amphioxus). Here the blastula 

 is invaginated at one point into the interior of the vesicle (Fig. 372, 2), 

 and progressively until the invaginated layer of cells comes in contact 

 with the opposite layer (3). At the same time the invagination-open- 

 ing becomes smaller and smaller. The stage in development thus 

 attained is designated gastrula. The external layer of cells is the 

 ectoblast (or epiblast), the internal layer the entoblast (or hypoblast). 

 The opening is designated the blastopore (or primitive mouth), and 

 the central space the primitive gut (archenteron). In vertebrates 

 the primitive mouth closes fully in the course of further development. 



FIG 372. 1-4, Formation of the hypoblast by imagination of the blastula, and the resulting gastrula from amphi 

 oxus (lancet-fish) ; - '- J f '- ' . . .. . 



blastc 



from 



(lancet-fish); 5, early and, 6, later development of the hypoblast by invaeination in petromyzon; , 

 >pore (primitive mouth); e, epiblast; ft, hypoblast in vertical section; 7, the ovum at this stage viewed 

 the side; , primitive mouth; r, spinal furrow (after Kupffer) 



Wholly identical gastrula-larvag are found in some radiates and worms that 

 move about independently in water and nourish themselves like the celenterates 

 through the primitive mouth. 



The formation of the hypoblast (h) by invagination in the situation 

 of the primitive mouth is exhibited distinctly in a similar manner by 

 the species of fish, the lampreys (petromyzon). In Fig. 372,5 and 6 illus- 

 trate these processes of formation in diagrammatic section, after Kupffer. 

 It will be observed that invagination takes place from the primitive 

 mouth (u) and thus the epiblast (e) and the hypoblast (h) are formed 

 in layers one upon the other, the primitive intestinal cavity being 

 situated beneath the hypoblast. These formations develop in much 

 the same manner also in batrachia. 



It appears justifiable to interpret the analogous early developmental 

 processes in mammals in a similar manner. According to van Bene- 

 den the ovule after segmentation is completed likewise exhibits two 

 layers of cells, the epiblast (Fig. 373 , /, e ) t which lies next the zona 



