GENERATION AND DEVELOPMENT. 



255 



Layers of the Blastoderm. Before long the blastoderm is found to 

 consist of three fundamental layers, Bpibla&t, Mesoblast, and Hypoblast. 



The way in which these are formed may be readily studied in a hen's 

 egg. In a freshly laid hen's egg, before incubation has commenced, the 

 blastoderm is found to consist of two layers (Fig. 407, S and D), the upper 

 of which forms a distinct membrane of columnar cells, while the lower 

 stratum consists of larger cells irregularly arranged. 



FIG. 407. Vertical section of area pellucida, and area opaca (left extremity of figure) of blasto- 

 derm of a fresh-laid egg (unincubated). ,S T , superficial layer corresponding to epiblast; >, deeper 

 layer, corresponding to hypoblast, and probably in part to mesoblast; M, large 'formative cells," 

 filled with yelk granules, and lying on the floor of the segmentation cavity ; A, the white yelk im- 

 mediately underlying the segmentation cavity (Strieker). 



Beneath the blastoderm there are a few scattered larger cells "for- 

 mative cells." In the lower of the above two layers, some cells become 

 flattened and unite to form a distinct membrane (hypoblast); the re- 

 maining cells of the lower layer, together with some of the large formative 



FIG. 408. Vertical section of blastoderm of chick (1st day of incubation). 8, epiblast, consisting 

 of short columnar cells; Z>, hypoblast, consisting of a single layer of flattened cells; M, "formative 

 cells." They are seen on the right of the figure, passing in between the epiblast and hypoblast to 

 form the mesoblast; A, white yelk granules. Many of the large "formative cells" are seen contain- 

 ing these granules (Strieker). 



cells, which migrate by amoeboid movement round the edge of the hypo- 

 blast (Fig. 408, M), constitute a third layer (mesoblast). 



These important changes are among the earliest results of incubation. 



From the epiblast are ultimately developed the epidermis and its various 

 appendages, also the cerebro-spinal nerve-centres, the sensorial epithelium 

 of the organs of special sense (eye, ear, nose), and the epithelium of the 

 mouth and salivary glands. 



From the hypoblast is developed the epithelium of the whole digestive 

 canal, together with that lining the ducts of all the glands which open into 

 it; also the glandular parenchyma of the glands (e.g., liver and pancreas) 

 connected with it, and the epithelium of the respiratory track. 



From the mesoblast are derived all the tissues and organs of the body 

 intervening between these two, the whole group of the connective tissues, 



