. Outlines of zoology. Zoology. REPRODUCTIVE SYSTEM. 467 often been suggested that the original Vertebrate animals were hermaphrodite. The quantity of yolk present in the egg varies very greatly in Vertebrates, and its presence or absence exercises a profound influence upon the processes of development. Following Hertwig, we may notice that the presence of yolk has both a physiological and a morphological effect. Physiologically, the presence of a store of nutriment enables the developmental process to be carried on uninterruptedly, and the period of independent life to be postponed until more
. Outlines of zoology. Zoology. REPRODUCTIVE SYSTEM. 467 often been suggested that the original Vertebrate animals were hermaphrodite. The quantity of yolk present in the egg varies very greatly in Vertebrates, and its presence or absence exercises a profound influence upon the processes of development. Following Hertwig, we may notice that the presence of yolk has both a physiological and a morphological effect. Physiologically, the presence of a store of nutriment enables the developmental process to be carried on uninterruptedly, and the period of independent life to be postponed until more or less complexity of organisation has been attained. Morphologically, the yolk acts as a check to the activity of the protoplasm, and by substituting an embryonic mode of nutrition for that for which the adult organism is fitted, tends to prevent a speedy establishment of the adult form. When much yolk is present, it usually forms a hemia-like yolk-sac, hang- ing down from the embryonic gut. As a further consequence, we may notice the tendency to the production of embryonic organs useful only during embryonic life. We must consider the formation of an organic connection between mother and un- born young as a further step in the same direction as the acquisition of yolk. This is hinted at in some Fishes and Reptiles, but culminates in the placental Mammals. It may be looked at in two different ways. On the one hand, the diversion of the nourishment from the ovary, FlG> dunng the period ot gestation, tends After Hertwig. to starve the remaining ovarian ova, , „. ' , ... , , . r 5?.., . r ,, Ovum;/.. follicular capsule \ fz., and this check to fertility is further foUicle cells \/.c, follicle cells foV prolonged during lactation (Ryder); ing discus proligerus; /.I., cavity on the other hand, the chance of occupied by liquor folliculi. survival is much increased, and the maternal sacrifice finds its justification in the increased specialisation o
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Keywords: ., bookauthorth, bookcentury1800, bookdecade1890, booksubjectzoology
