SE 
oO 
2 
EARLIEST STAGES OF DEVELOPMENT OF OVUM. 561 
_perature that might be fatal to the Hydrz themselves. The 
same thing is observable among the Rotifera; 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 gemmz ; 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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