THE DEVELOPMENT OF THE VEETEBEATA. O 



the amount of heat, in excess of the ordinary temperature 

 of the air, which is required, from its own body, by the 

 process of incubation. 



The first step in the development of the embryo is the 

 division of the vitelline substance into cleavage-masses, of 

 which there are at first two, then four, then eight, and so 

 on. The germiaal vesicle is no longer seen, but each 

 cleavage-maSs contains a nucleus. The cleavage-masses 

 eventually become very small, and are called embryo-cells, 

 as the body of the embryo is bxiilt up out of them. The 

 process of yelk-division may be either complete or partial. 

 In the former case, it, from the first, affects the whole 

 yelk ; in the latter, it commences in part of the yelk, and 

 gradually extends to the rest. The blastoderm, or em- 

 bryogenic tissue iu which it results, very early exhibits 

 two distinguishable strata — an inner, the so-called mucous 

 stratum (hypoblast) , which gives rise to the epithelium of 

 the alimentary tract; and an outer, the serous stratum 

 (epiblastj, from which the epidermis and the cerebro-spinal 

 nervous centres are evolved. Between these appears the 

 intermediate stratum fmesoblastj , which gives rise to all the 

 structtu-es (save the brain and spinal marrow) which, in the 

 adult, are included between the epidermis of the integu- 

 ment and the epithehum of the alimentary tract and its 

 appendages. 



A linear depression, the primitive groove (Fig. 2, A, c), 

 makes its appearance on the surface of the blastoderm, and 

 the substance of the mesoblast along each side of this groove 

 grows up, carrying with it the supei'jacent epiblast. Thus 

 are produced the two dorsal lamince, the free edges of which 

 arch over towards one another, and eventually xxnite, so as 

 to convert the primitive groove into the cerebro-spinal canal. 

 The portion of the epiblast which lines this, cut off from 

 the rest, becomes thickened, and takes on the structure of 

 the brain, or Eiiceplialon, in the region of the head; and 

 of the spinal cord, or Myelon, in the region of the spine. 

 The rest of the epiblast is converted into the epidermis. 



