EARLIEST STAGES OF DEVELOPMENT OF OVUM. 561 
perature that might he fatal to the Hydrse themselves. The 
same thing is observable among the Rotifer a; for, as has 
long been known, two kinds of eggs are produced by them, 
the ordinary and the “winter eggs ;” and it now appears that 
the ordinary eggs, being evolved without any generative pro¬ 
cess, and with a rapidity proportional to the favouring 
influences of food and warmth, are really to be regarded as 
internal gemmae; whilst the “ winter eggs,” which are pro¬ 
duced in the autumn by the concurrent action of males and 
females, and have a peculiarly dense horny investment, are 
the only true ova. Among the Aphides (§ 746), again, it has 
been experimentally shown that the non-sexual multiplication 
may be indefinitely protracted by warmth and food ; whilst a 
reduction in the temperature and in the supply of nutriment 
causes this at any time to give place to sexual generation. 
736. The first obvious change that presents itself in the 
Ovum, after its fertilization, is the “ segmentation,” or division 
of the yolk-mass into two halves, by the formation of a sort 
of hour-glass contraction, which gradually deepens, until it 
produces a complete separation. Another segmentation of 
these two halves soon follows in the opposite direction, so 
that the yolk-mass becomes divided into four segments ; each 
of these in its turn undergoes the like subdivision; and this. 
duplicating process is repeated, forming successively 8, 16*. 
32, 64, &c., segments, until a “mulberry-mass” is produced, 
which is composed of an aggregation of an immense number- 
of minute yolk-spherules. Up to this stage, the develop¬ 
mental process takes place on essentially the same plan in all 
animals, save that in some the process of segmentation does 
not extend to the entire mass of the yolk, but only to a small 
proportion of it, which is distinguished as the “germ-yolk,” 
whilst the remainder, which is applied to the nourishment of 
the more advanced embryo through an entirely different chan¬ 
nel, is known as the “food-yolk” (§ 754). 
737. It appears, among some of the simplest Worms, as if 
the “mulberry-mass” gradually shaped itself into the body 
of the animal, without the intervention of any intermediate 
structure ; but in almost all Animals, the first stages in deve¬ 
lopment tend to the production of a membranous expansion 
that may be likened to the “ cotyledon ” of Flowering Plants,— 
with this important difference, however, that whilst the latter 
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