844 PHYSIOLOGY. 



To fully appreciate the different stages of the process of repro- 

 duction mentioned above, a brief account of the origin, formation, 

 and structure of the ovum and spermatozoon is essential. 



The beginning of the differentiation of the organs of the animal 

 body from the blastoderm, of which we will speak later, we find 

 expressed in the arrangement of the building material, so to speak, 

 in three distinct so-called germinal layers an outer layer, the ecto- 

 derm; an inner one, the entoderm; and between these two a middle 

 layer, the mesoderm. The first two layers we find very well defined 

 in all Metazoa, and from them all vital organs of the body, composed 

 of epithelium, are developed. The middle layer supplies the sup- 

 porting and connective tissues and the vascular system. In the 

 lower types of Metazoa, which require very little supporting material, 

 and in which a special vascular system is not present, we find also 

 the mesoderm very scantily presented. The higher the type of the 



Fjg. 380. Transaction of Chick Embryo, Showing the Three 

 Plastodermic Layers. (MANTON.) 



animal, the more we find the mesoderm developed, until we finally 

 see it not only as a well-defined single layer, but it becomes split 

 into two secondary layers : one, the parietal mesoderm, which follows 

 and gives support, and supplies blood-vessels and nerves to the 

 ectoderm and its derivatives, and a second one, the visceral mesoderm, 

 which acts in the same way for the entoderm. The space formed 

 between them constitutes the future body cavity or ccelom. It is 

 in this middle layer where we find the first traces of the two kinds 

 of the elements of reproduction the spermatozoa and ova. In the 

 lower types, with scanty mesoderm, we find these elements loosely 

 scattered within it; in the higher types, with well-defined, double- 

 layered mesoderm, we find certain parts of it crystallized, so to 

 say, as organs of reproduction the ovaries and testicles. It was 

 Waldeyer who first called attention to the fact that a certain part 

 on the visceral layer of the mesoderm becomes thickened and forms 

 the so-called genital ridge,, which gives rise to the organs of the 

 primitive genito-urinary apparatus. A part of the ridge-cells be- 



