THE CELL AND THE TISSUES. 23 



favorable conditions the spermatozoa reach the ovum, when a single 

 element penetrates the envelopes of the egg and is received within 

 the protoplasm of the female cell. The entrance of the spermatozoon 

 causes a new disturbance within the ovum, resulting in the formation 

 of the male pronucleus. Subsequently the latter joins with the 

 female pronucleus, the fusion of the two pronuclei being followed by 

 a temporary disappearance of all nucleus within the ovum. Shortly 

 afterwards the new nucleus of segmentation appears, so called 

 from the fact that within this body cleavage of the ovum is first 

 established. 



The process of segmentation following the fertilization of the 

 ovum is essentially one of indirect cell-division, in which the 

 stages, although modified in certain details, are essentially the same 

 as those already described. The mammalian ovum undergoes a 

 total segmentation; although the resulting segments are, strictly 

 regarded, not quite equal in size, yet, as a matter of simplicity, they 

 may be regarded as such, and the division characterized as total 

 equal segmentation. 



The repeated cleavage of the segmentation-spheres into which 

 the ovum is divided soon produces a mass of innumerable cells con- 

 stituting the blastoderm ; the latter, by continued division and 

 further differentiation, subsequently gives place to a cell-area, in 

 which at first two layers, an outer and an inner, and later a third 

 middle stratum, of cells appear. These more or less imperfectly 

 defined tracts constitute the important primary blastodermic 

 layers, the ectoderm, mesoderm, and entoderm, from which are 

 derived all the tissues of the body. The reader must be referred to 

 the various text-books of embryology for a detailed account of the 

 complicated and often obscure processes of maturation, fertilization, 

 segmentation, and blastulation, of which only the most salient points 

 have been indicated above. 



THE TISSUES. 



Every tissue is composed of two parts, the cellular elements and 

 the intercellular substance. Upon the first of these depends the 

 vitality of the tissue, while its physical properties are determined by 

 the character of the second. The physical condition of the inter- 

 cellular substances includes a wide latitude, varying from that of a 

 fluid, as blood or lymph, through all degrees of density, until, by 

 the additional impregnation of calcareous matters, the well-known 

 hardness of bone or dentine is attained. 



The proportion between the cellular elements and the intercellular 

 substance of mesodermic tissues varies with age and development, 

 the intercellular substance in the early stages being scanty and very 



