LAYEES OF THE BLASTODERM. 683 



that still later in this disc, when expanded and altered in shape, there 

 is formed the first trace of the embryo. The same distinction, how- 

 ever, as in birds, a])pears in the end between a transparent or embryonal 

 area, and an opaque peripheral area, a part of which is occupied by 

 the vessels of the first circulation. 



In birds the blastoderm spreads itself rapidly during the first stages 

 of incubation by cell-multiplication over the surface of the yolk, until 

 at last the whole is covered by its layers ; but in mammals, as the 

 yolk is still of comparatively small size after segmentation is complete, 

 but undergoes soon afterwards very rapid and great enlargement, and 

 as it is completely covered by the primitive blastoderm, it is obvious 

 that that membrane must undergo corresponding extension, not by 

 marginal, but by interstitial cellular multiplication. 



Trilaminar structure. — The bilaminar blastoderm which results 

 directly from segmentation soon undergoes farther changes, by which a 

 third most important element is introduced into its composition, so 

 that, at an early period of development and previous to the actual 

 formation of any part of the embryo, it is found to consist of three 

 layers of cellular elements, placed one above the other. These layers 

 may, from their relative position on the yolk, be named the outer, 

 middle, and inner blastodermic membranes, ectoderm, mesoderm, and 

 cndoderm, or, following the nomenclature of Foster and Balfour, opihiasf, 

 mesotlast, and hypoMast, the upper, middle, and lower germs ; and the 

 ovum of birds and mammals may thus, along with that of a consider- 

 able number of animals, be styled triplohlasfic. 



The origin of the middle layer is still involved in some obscurity. 

 By one set of observers it is considered to be most closely connected 

 with the original lower layer, and while the original upper layer of 

 the primitive bilaminar blastoderm remains undivided, constituting 

 the epihiast, the looser substance of the original lower layer undergoes 

 a differentiating change, by which there is separated from its under 

 part a thin lamina of flattened united cells to form the Injpohlast, while 

 the remaining portion, with rounded cells of a different structure, 

 becomes distinct superiorly, and accumulates between the upper and 

 lower layers, especially towards the centre, to form the foundation of 

 the mesq lfkish which according to this view would result, like the 

 epiblast and hypoblast, from the primary segmentation. But by other 

 embryologists, it is held that a part, if not the whole, of the mesoblast 

 proceeds from a secondary process of segmentation or cell formation 

 occurring below the original blastoderm ; and farther, that the new 

 cells which thus give rise to the mesoblast are carried from below 

 towards the place where they form that layer by migratory movements, 

 the nature of which is not yet understood." 



Leaving the question of the origin of the middle layer for farther 

 remark hereafter^fe^-e shall here state in the shortest and most general 

 terms the relation ascertained to subsist between the three several 

 constituents of the organised germ and the origin of the rudiments of 

 the embryo and other parts developed from the ovum. In doing so, if 

 allowance be made for the differences previously noted, the same 

 description may apply to the fundamental formative processes of birds 

 and of mammals. 



Relation of the Layers to Development. — With respect to the 

 histogenetic changes which accompany the conversion of parts of the 



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