III.] THE MESOBLAST. 45 



first completed in the centre of the pellucid area, we shall 

 henceforward speak of as the hypoblast. 



Between it and the epiblast many of the cells of the 

 original lower layer are enclosed, and in addition some of the 

 formative cells (migrating by help of amoeboid movements 

 after the fashion of white-blood corpuscles) begin to travel 

 round the edge of the hypoblast, and to pass in between it 

 and the epiblast. 



The cells, whether originally "formative" cells or cells 

 from the lower layer, thus gathered between the epiblast and 

 hypoblast, undergo a process of endogenous cell-formation,by 

 which the whole of the interior of each becomes converted 

 into a number of new cells. These new cells, spherical in 

 form, and possessing a large nucleus with a distinct nucleolus, 

 are first formed in the centre of the pellucid area and sub- 

 sequently in its periphery. They constitute the third layer 

 or mesoUast (Fig. 10, B). 



The epiblast is the Bornblatt (corneal layer), and the hypoblast the Darm- 

 drilsenblatt (epithelial glandular layer) of the Germans, while those parts of the 

 mesoblast which take part in the formation of the somatopleure and splanchno- 

 pleure correspond respectively to the Haiit-mushel-platte and Larvi-faser-platte. 



All blood-vessels aiise in the mesoblast. Hence the vascular layer of the 

 older writers fall entirely within the mesoblast. 



The serous layer of the same authors includes the whole of the epiblast, but 

 also comprises a certain portion of mesoblast ; for they speak of all the organs of 

 animal life (skin, bones, muscle, &c.) as being formed out of the serous layer, 

 whereas the epiblast proper gives rise only to the epidermis and to certain 

 parts of the nervous system. In the same way their mucous layer corresponds 

 to the hypoblast with so much of the mesoblast as takes part in the formation of 

 the organs of organic life. Their vascular layer therefore answers to a part only 

 of the mesoblast, viz. that part in which blood-vessels are especially developed. 



It is worthy of notice that the cells of the epiblast are themselves the 

 direct results of segmentation ; but that the hypoblast and mesoblast are 

 formed at a subsequent period, and are therefore only indirectly the results of 

 segmentation. The true difference between the hypoblast and mesoblast lies 

 in'the mode in which each layer is formed, and not in any essential differ- 

 ence in the segmentation-spheres from which each is derived; 



At about the time when the hypoblast is completely 

 formed as a distinct membrane, the mesoblast cells form a 

 somewhat thick mass in the centre of the blastoderm, and 

 cause the central opacity spoken of above as the embryonic 

 shield. 



4. Soon after this, between the 8th and 12th hours, the 

 hitherto circular pellucid area becomes oval (the opaque 

 area remaining circular) . The oval is, with remarkable regu- 



