090 GENERAL PHENOMENA OF EMBRYONIC DEVELOPMENT. 



embryonic area may be given. From this area, as from a centre, the 

 changes of development in some measure emanate or spread towards the 

 periphery. In birds the central area is from the first distinguished 

 from the surrounding part by greater transparency and thinness of the 

 bUistoderm, and thus (as ah-eady described) arises the distinction of the 

 transparent and opaque areas. In mammals, on the other hand, the 

 germinal part of the blastoderm is at first entirely opaque, forming the 

 embryonic disc of Costc, Bischoff, and others ; and it is by a subsequent 

 change that a part of this disc clears up or becomes thin and transparent, 

 and that an opaque area is formed in the peripheral part. In both 

 birds and mammals the embryonic area, from being simply round at 

 first, becomes soon somewhat pyriform, and subsequently oval or con- 

 tracted in the centre, like the body of a violin. 



Fig. 500. — First appearance oi? the PRiMiirvE Trace and MEDULLAur Canal in 

 THE Ovum op the Dog (from Bischofi'). 



«, Jj, and c, represent the natural size of the ova of wliicli tlie several germinal area; 

 are represented in A, B, and C. In A the germinal area is pyriform, and the primitive 

 trace occupies two-thirds of the narrow hinder end. In B the trace is elongated and 

 on the two sides are the raised medullary plates, mp, with the primitive groove between. 

 In C the distinction between transparent area, at, and opaque ai-ea, ao, is marked by the 

 outline. 



It is in the hinder narrower part of this embryonic area, when it has 

 assumed the pyriform shape, that the earliest trace of the embryo can 

 be discerned. This forms the well known primitive streak and groove 

 of authors, but it appears from the observations of Dursy and Balfour 

 in the chick, and of Hensen in the rabbit, that the primitive trace and 

 groove, which are the first indications of embryonic formation, are only 

 transitory and evanescent, and that they are succeeded by the medullary 

 groove and dorsal plates, which commence beyond the cephalic end of 

 the primitive trace, and grow backwards towards the caudal extremity, 

 so as gradually to thrust out as it were at the end the shrivelled 

 remains of the primitive trace. The anterior extremity of this medullary 

 groove becomes afterwards the cephalic, and the posterior extremity or 

 that towards the primitive trace becomes the caudal part of the cranio- 

 vertebral axis of the embryo. 



