GENERAL INTRODUCTION. 



characteristics, or stirp, of the parent. The study of the internal changes accompanying fertilization therefore leads 



directly to an inquiry into the mechanism of inheritance, and our study of these changes cannot be too precise or detailed. 



The first decisive discovery regarding the internal phenomena of fertilization was made by Oscar Hertwig,' in 1875, 



in the egg of the sea-urchin. Hertwig determined the fact, namely, that the nucleus of the spermatozoon, or "sperm- 



H 



K 



Fig. I. 



Fig. I. — Cleavage or segmentation of the ovum of the sea-urchin Toxopneustes variegattis. Ag. From the Uving object. Enlarged 365 diameters. 



A. The unripe ovarian egg, showing the large nucleus or germinal vesicle (GV) containing the nucleolus or germinal spot (OS), and traversed by a delicate network of chromatin. 



B. Ovarian egg half an hour after the formation of the polar bodies (PB). The egg-nucleus (E) is now much reduced in size. 



C. Four minutes after entrance of the spermatozoon. Union of the minute sperm-nucleus (shown as a black dot) with the egg-nucleus (E); C. The " entrance-cone " at the point 



where the spermatozoon has entered. 



D. " Pause " twenty minutes after entrance, showing the cleavage-nucleus, which has resulted from the union of the egg-nucleus and sperm-nucleus. The sperm-aster has divided to form 



two asters lying at opposite poles of the nucleus. 



E. First cleavage, forty-five minutes after fertilization, showing the amphiaster, spindle, and the daughter-nucleus resulting from the division of the cleavage-nucleus. 



F. Two-cell stage, fifty minutes, showing the daughter-nuclei, each accompanied by an aster. 



U. .Second cleavage, one hour, six minutes. H. Four-cell stage resulting. 



I. Four-cell stage from the side, showing the asters already divided for the following cleavage. J. Eight-cell embryo, one hour, thirty-eight minutes. 



K. Sixteen-cell embryo, two hours. At the lower side are four small cells, the " micromeres," that mark the first visible differentiation in the embryo. 



L. Blastula stage, about three hours. The cells or blastomeres are now arranged so as to surround a central cavity, the blastocoel. 



nucleus" unites with the "egg-nucleus" to form a single "cleavage-nucleus," which is the parent of all the nuclei of the 

 embryo. This discovery, soon extended to other animals and to the plants as well, gave rise to the view, advocated by 

 Hertwig, Strasburger, Kolliker, Weismann, and many others, that the nuclear substance, or chromatin (so named by 

 Flemming), is the most essential element in the germ-cell and must be regarded as the physical basis of inheritance. 



^Beitrage zur Kenntniss der Bildung, Befruchtung und Theilung des Thierischen Eies. I. Morph. Jahrb., 1875. 



