EARLIEST STAGES OF DEVELOPMENT OF OVUM. 561 



perature that might be 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, !(> 

 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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