CHAP, x.] THE DEVELOPMENT OF THE CAT. 329 



uterine cavity, and conies away, bringing with it a portion of the 

 maternal part of the placenta, together with the foetal part, with 

 which the maternal part is inextricably united. Thus the super- 

 ficial part of the decidua serotina comes away, while its deeper part 

 is left, with a torn and bleeding surface. Coincidently with the 

 termination of gestation, the mammary glands take on functional 

 activity and become ready to play their part in the post-natal 

 development of the young. After the expulsion of the embryos, of 

 which several are in general simultaneously developed, the uterus 

 forms a fresh internal lining, while the thickness of its walls 

 decreases by degeneration and absorption of a portion of its muscular 

 tissue, which had so much increased in quantity during pregnancy. 



7. It remains to consider seriatim, the development of the 

 various TISSUES and ORGANS of the body. 



All the various tissues and structures of the adult cat (connective 

 tissue, cartilage, bone all parts of the skeleton of course included) 

 arise from the primitive fluid, granule, and cells of the fertilized 

 ovum by a process which is called differentiation. This term, which 

 is often used as if it were a real explanation, simply denotes the 

 fact that the various parts arise not through external actions, which 

 are but the concomitants and conditions of their origin, but by an 

 as yet utterly inexplicable and innate power possessed by the primi- 

 tive substance or matrix, within which the parts referred to, come 

 (under the requisite external conditions) gradually to manifest 

 themselves. Other terms relating to development similarly denote 

 spontaneous and mysterious actions of the formative power, and are 

 but convenient phrases for denoting the actions of such power, and 

 not explanations of it. 



Such terms, for example, are segmentation which denotes that a 

 structure, primitively of one piece, spontaneously divides its substance 

 into parts ; vacuolation denoting the spontaneous resolution of part of 

 a more or less dense structure in such a way as to give rise to a cavity 

 or cavities within it ; and fencstration denoting that a solid structure 

 has dissolved itself at one spot or more, so as to give rise to an 

 aperture perforating it. "When then the development of the tissues 

 and parts of the body are herein described, the intention is but to 

 state the order and mode in which they manifest themselves, the 

 fact being distinctly recognized that an innate force is the real and 

 efficient cause. 



The primitive almost fluid substance containing granules, which 

 exists in the developing ovum, is known as protoplasm ; and proto- 

 plasm is often spoken of as if it were a sort of primary organic material 

 a distinct kind of formed substance from which all organisms arise. 

 But the fact that the primitive substance of one animal or plant is 

 not to be distinguished by any chemical or physical test we can apply 

 from the primitive substance of another animal or plant, does not by 

 any means prove that the two are really the same substance. It does 

 not prove this identity, because of the very different results which 

 are successively evolved in the two cases, as development proceeds. 



