330 Mr. H. J. Carter on the 



power, and seek for the successive duplicative subdivisions of 

 the yelk, from the first to that whicli appears to be the last 

 duplicative subdivision (fig. 1, 7i/?7^), 'since, although it is easy 

 enough to determine these divisions from the first to the third 

 degree inclusively, in which the yelk becomes divided into 

 eight cells (fig. 6), the other degrees (figs. 7-10) must be 

 judged of by the comparative size of the cells ; and thus all 

 the rest of the illustrations of this duplicative subdivision have 

 been selected in this way, from the fourth (fig. 7) to the 

 seventh (fig. 10) inclusively. 



If, on the other hand, the ovum, while undergoing this du- 

 plicative subdivision, be isolated and compressed under a glass 

 cover and ^-incli compound power, a nucleus and nucleolus 

 will be observed, as figured in the illustration of the first de- 

 gree of duplicative division, viz. that in which the yelk is 

 divided into two cells only (fig. 4, dd) . But, as this compres- 

 sion &c. cannot be can-ied into effect when the ova are viewed 

 m situ and en masse (as in fig. 1), the nucleus cannot be then 

 seen for want of sufficient light, and therefore is not intro- 

 duced into the other figures, although it must be assumed 

 that, whenever a cell undergoes division, the nucleus does so 

 also, and that if these cells, however much subdivided, could 

 be brought under sufficient compression, and thus rendered 

 sufficiently transparent, each would be found to contain a 

 nucleus. 



There is also another featm-e which characterizes this period, 

 viz. that the ovum is without colour^ Avhile very soon after the 

 ultimate duplicative subdivision of the yelk is reached, and 

 the yelk begins to be elongated into the form of the embryo 

 (figs. 11 & 12), the latter presents the same kind of red- violet 

 coWr as the spongozoa. 



Striking, however, as this distinction is, the ciliated ecto- 

 derm, which becomes such a prominent feature of the embryo, 

 is developed over the yelk while the ovum is still colourless 

 and apparently has not begun to elongate itself into the em- 

 bryonic form. 



It is thus interesting to find that the yelk in the lowest 

 undergoes the same kind of segmentation as in the highest 

 animals, as if in all this preliminary process were absolutely 

 necessary to the further evolution of the new being. Nor is 

 it a little interesting to me to find also that my drawings of 

 it to-day in the sponge correspond exactly with those which 

 I made in 1837 from the ova of the freshwater newt, when 

 as a student, aided by my dear friend Dr. Sharpey, I followed 

 tliroughout the development of this reptile in ova obtained 

 from a pool in the " Regent's Park." 



