■jiu: (KLL 03 



germ layers. The outer of these is known as the ectoderm or epihlasl, 

 the inner as the entoderm or Jiypohlast. Between these two layers 

 and derived from them a third layer is formed, the mesoderm or meso- 

 blast. These three layers constitute the blastoderm. 



As to what determines and controls fertilization, comparatively little is 

 known. As in ordinary mitosis, the origin of the centrosome is obscure. In 

 some forms, at least, the centrosome of the spermatid enters into the formation 

 of the middle piece of the spermatozoon. The male centrosome thus enters the 

 ovum. It is also known that in some eggs the cgg-centrosome disappears soon 

 after the extrusion of the second polar body, and that the centrosome of the fer- 

 tilized egg develops in close relation to the middle piece of the spermatozoon. 

 These facts point to the male centrosome as the centrosome of fertilization. 



d 

 Fig. 14. — ^The Two Primary Germ Layers; from transverse section through primitive 

 groove of a chick of 27 hours' incubation, a, Ectoderm (outer germ layer); h, ento- 

 derm (inner germ layer); c, mesoderm (middle germ layer); d, anlage of notochord. 



The ovum and spermatozoon are apparently brought together bj^ a definite 

 attraction on the part of the ovum toward the spermatozoon. The nature of 

 this attraction is unknown. It is possibly chemical, and is exerted only be- 

 tween ova and spermatozoa of the same species. This has been proved for lower 

 forms by mixing ova of one species and spermatozoa from several species in an 

 inert medium when only spermatozoa of the same species will attach themselves 

 to the ova. That the attractive force lies in the cytoplasm is shown by the fact 

 that small pieces of the egg protoplasm free from nuclear elements will exert 

 sufficient powers to cause spermatozoa to enter them. 



As to the point of entrance of the spermatozoon, some eggs may be entered 

 at any point, others are permeable at but one point. 



One spermatozoon only is required for fertilization, and when this sper- 

 matozoon has entered, the egg apparently loses its power of attracting sper- 

 matozoa, or else develops some actual defense against further entrance of 

 spermatozoa. 



TECHNIC 



1. Fresh cells may be studied by gently scraping the surface of the tongue, 

 transferring the mucus thus obtained to a glass slide and covering with a cover- 

 glass. 



2. Red blood cells from the frog are prepared as follows: After killing the 



