[492] G. 'Swirski, Untersuch. üb. d. Entwick. d. Schultergürtels u. d. Skelets d. Brustflosse d. Hechts. Inaug. Diss. Dorpat, 1880.
[493] The horny fibres are mesoblastic products; they are formed, in the first instance, as extremely delicate fibrils on the inner side of the membrane separating the epiblast from the mesoblast.
[494] In one example where the metapterygium had 13 rays the mesopterygium had only 2 rays.
[495] Loc. cit. p. 514.
[496] I employ here the same letters to indicate the stages as in my “Monograph on Elasmobranch Fishes.”
XXI. On the Evolution of the Placenta, and on the possibility of employing the characters of the Placenta in the Classification of the Mammalia[497].
From Owen's observations on the Marsupials it is clear that the yolk-sack in this group plays an important (if not the most important) part, in absorbing the maternal nutriment destined for the fœtus. The fact that in Marsupials both the yolk-sack and the allantois are concerned in rendering the chorion vascular, makes it à priori probable that this was also the case in the primitive types of the Placentalia; and this deduction is supported by the fact that in the Rodentia, Insectivora, and Cheiroptera this peculiarity of the fœtal membranes is actually found. In the primitive Placentalia it is also probable that from the discoidal allantoic region of the chorion simple fœtal villi, like those of the Pig, projected into uterine crypts; but it is not certain how far the umbilical region of the chorion, which was no doubt vascular, may also have been villous. From such a primitive type of fœtal membranes divergencies in various directions have given rise to the types of fœtal membranes found at the present day.
In a general way it may be laid down that variations in any direction which tended to increase the absorbing capacities of the chorion would be advantageous. There are two obvious ways in which this might be done, viz. (1) by increasing the complexity of the fœtal villi and maternal crypts over a limited area, (2) by increasing the area of the part of the chorion covered by the placental villi. Various combinations of the two processes would also, of course, be advantageous.
The most fundamental change which has taken place in all the existing Placentalia is the exclusion of the umbilical vesicle from any important function in the nutrition of the fœtus.