CS4 THE OVUM AND BLASTODERM. 



blastoderm into the several organs and textures, the reader is referred 

 to the various parts of the section on General Anatomy in -which the 

 development of the textures is treated of. Here it is enough to state, 

 that in the upper layer or epiblast of the bird's ovum it is mainly 

 by endogenous cell-multiplication that the increase of substance and 

 extension of area is effected ; that in the lower layer, there is, accord- 

 ing to Foster and Balfour, continued conversion of the cells of tho 

 white yolk into those of the hypoblast ; and that in the mesoblast 

 there is prolonged addition of cellular elements by new production of 

 formative cells from below the germ ; and further, that in all the three 

 layers it is mainly by internal differentiation of the various groups of 

 the cells so formed that are produced the different kinds of formative 

 bases, or initial deposits, whether cellular or extra -cellular, which are 

 converted by farther changes into the rudiments of the several organs 

 and textures of the animal body or its foetal appendages. But, while 

 the formative processes consist essentially in minute histogenetic 

 changes, they are also accompanied by changes of form which are 

 more obvious. Thus the folding or inflection of certain of the 

 layers of the blastoderm which brings about the enclosure of the 

 visceral cavity of the bod\^, or that which accompanies the formation 

 of the amnion ; the progressive rising of the dorsal laminse and their 

 final union, which attends the closure of the canal for the brain and 

 spinal cord ; the increased accumulation of formative cells in one place 

 leading to growtli and increase, and their diminution or removal in 

 others leading to atrophy ; the fusion of certain membranes or masses 

 of tissue uniting parts which were previously separate, and the fission 

 or solution of continuity between other masses producing their separa- 

 tion ; the excavation of one set of hollows and the obliteration of 

 others, as in the case of blood-vessels and ducts, — are only a few 

 examples of developmental changes, which are dependent, no doubt, 

 more immediately on textural differentiation, but which indicate different 

 forms and modes in which the constructive processes are brought 

 --about. 



The following is the general relation of the several germinal layers 

 to the production of different systems and organs of the embryo and its 

 accessory parts in so far as yet discovered. 



1. From the qnblast proceed the epidermis and its appendages, the 

 great nervous centres, and the principal parts of the eye, ear, and 

 nose ; one la^'cr of the amnion and yolk-sac, and in mammals, probably 

 the outer layer of the permanent chorion. 



'2. From the lijipoUast proceed the epithelial lining of the whole 

 alimentary canal (excepting that of the mouth), and of the lungs, the 

 epithelial lining of the ducts of the glands connected with the ali- 

 mentary canal, and also the deep layer of the yolk-sac and allantois. 



3. From the mesoblast proceed in general all the parts of the 

 skeleton, the muscles, fascia, and tendons, the peripheral nerves, the 

 true skin, the connective tissue, the vascular system and blood, the 

 muscular and fibrous coats of the alimentary canal and all other 

 visceral passages, the serous membranes, the parenchyma of many 

 glands, and the genito-urinary system, together with the outer layer 

 of the amnion, the vascular layers of the yolk-sac, the allantois and 

 _the chorion, and the foetal part of the placenta. 



The mesoblast does not, however, serve as the basis of these very 



