MICHIGAN ACADEMY OF SCIENCE. 63 



the tube the nuclei slip out of the cytoplasm so that it is only as 

 naked nuclei that they enter the embryo-sac, and iei'tilization is 

 accomplished by the entry of one of these naked nuclei into the 



It is clear then that the nucleus is the most important structure 

 in sexual reproduction, at least so far as the male cell is concerned. 



Further consideration of the process shows that in the lower 

 forms, where the union is that of whole cells, it is not merely the 

 cytoplasm but also the nuclei that unite. The latter is the case 

 also in the higher forms. Sexual reproduction, then, is not merely 

 the union of cells, or the entry of a male nucleus into a female 

 cell, but the union of the two nuclei. 



But this union of cells and nuclei is not all of the process. To 

 reproduce there must be cell division again, whether it be to pro- 

 duce new individuals at once, in the case of one-celled plants or 

 animals, or to produce the many cells of which the new individual 

 consists, in the case of the many-celled plants and animals. 



It will be necessary to review hastily the process of cell and 

 nuclear division in order to understand more clearly what effect 

 the union of nuclei has on the subsequent process. 



In its essentials the mitotic division of the cell consists of the 

 division of all elements of the protoplasm into like halves, and the 

 regeneration by each half so formed of the missing half. This latter 

 point is as important as the former. Thus the cytoplasm divides 

 into two masses of cytoplasm, the plastids, in many cases at least, 

 into two plastids each, etc. This division occurs either through 

 cleavage due to the formation of vacuoles between the two halves- 

 to-be or through a drawing apart of the two halves and gradual 

 pinching off as happens when a drop of glue drops from a stick 

 or other object. 



The nucleus is not a simple drop of slightly different protoplasm 

 but is more complicated in its structure and accordingly in its 

 mode of division, although it conforms to the rules formulated 

 above. It is a vacuole (nuclear sap) surrounded by a thin, tough 

 membrane (nuclear membrane, which is a part of the cytoplasm) 

 with a tangled thread suspended in the vacuole. On the thread, 

 which may have cross connections from one loop to the next, there 

 are irregular lumps of a highl}^ staining protoplasm particularly 

 at the intersections of the threads. These lumps are made of a 

 substance or substances to which the collective name chromatin is 

 given. There is usually a store of reserve material as a large drop, 

 the nucleolus. This is proteid in nature and is clearly closely re- 

 lated in composition to the chromatin. 



