1008 EMBRYOLOGY. 



appearance of the middle layer, and although the embryo at a later period reveals 

 its first outlines in this part of the blastoderm, it must not be forgotten that it 

 is not yet traced ; so that the qualification of embryonal area cannot be accepted 

 in a literal sense. 



In the following days, the internal and middle layers continue to grow. By 

 multiplication of the elements of the mesoderm, the germinal area increases in 

 thickness and in surface, pushing up the ectoderm, which becomes salient like a 

 shield, and has a well-defined circular outline. The germinal area at this moment 

 reveals the thickening of the central region of the mesoderm (mesoblast), which 

 extends by its borders at the same time as the endoderm, so that the walls of the 

 vesicle are soon composed of three layers throughout. 



From the seventh to the eighth day, the blastoderm figures as an elliptical 

 vesicle, somewhat flattened, from 7 to 8 mm. in diameter, filled with a trans- 

 parent albuminous fluid, and formed of three superposed layers. It does not yefc 

 exhibit any traces of morpliological differentiation except the germinal area, or 



Fig. 545. 



VIEWS OF THE BLASTODERMIC VESICLE OF A RABBIT ON THE SEVENTH DAY. 

 A. FROM ABOVE; B, FROM THE SIDE. 



ag, Embryonal area ; ge, limit of the gastrodisc or endoderm (hypoblast). 



central thickening of the mesoderm. It is true, however, that the outline of the 

 embryo soon becomes apparent, and the three layers of the blastoderm play a 

 distinct part in its formation, which it is necessary to indicate beforehand. 



a. The external layer — ectoderm, epiblasf — is also named the sensorj/ or 

 smsitive layer, because it gives rise to the epidermis or epidermic formation, and 

 to all the nerve-elements of the sensory terminations — retina, organ of Corti, 

 etc. ; it also furnishes the outline of the central nervous system. 



h. The internal layer — endoderm, ht/jwblast — also merits the name of mucous 

 layer, as it originates — not the mucous membranes, as is often said — but the 

 epithelium of the post-diaphragmatic portion of the digestive apparatus and the 

 glands annexed thereto —liver and pancreas. 



c. The middle layer — mesoderm, mesohlast — serves as a centre for all the other 

 tissues — connective, muscular, blood-vessels and lymphatics, serous membranes, 

 sexual glands, etc. The multiplicity of its future specializations does not, there- 

 fore, authorize a particular denomination, though it is at times designated as the 



