Januabt 14, 1916] 



SCIENCE 



45 



current until Kolliker in 1841 showed by 

 their development that they were modified 

 cells. Nevertheless, there was, strictly speak- 

 ing, no immediate application of these re- 

 sults to the problems of fertilization. 



The half century from 1824 to 1874 

 yielded relatively little advance in fertiliza- 

 tion theory ; the opinion that the spermato- 

 zoon actually penetrated into the ovum 

 gradually gained ground largely from the 

 very logic of the situation, but partly from 

 various observations. Bischoff's contact 

 theory, which was the only alternative, was 

 criticized because if the sperm does not 

 penetrate, but remains outside of the mem- 

 brane, there is absence of that direct con- 

 tact between sperm and egg substance pos- 

 tulated by the theory. Wagner's criticism 

 was also very effective; a ferment does not 

 determine the character of a reaction, but 

 the spermatozoon does, for it transmits pa- 

 ternal characteristics. In the way of ob- 

 servations Barry in 1840, Newport,' 1854^ 

 1855, Meissner, 1855, and others maintained 

 observations of penetration of the ovum by 

 the spermatozoon; Keber (1854) laid espe- 

 cial emphasis on the micropyle as adapted 

 for entrance of a spermatozoon. These ob- 

 servations were on the whole inconclusive, 

 for actual penetration was not observed, 

 but inferred from the presence of spermato- 

 zoa inside the egg membrane. Moreover, 

 the spermatozoon could not be discovered 

 within the egg. 



The Modern Period. — The preceding pe- 

 riod (1824^1874) was coincident, as we 

 have seen, with the early history of the cell 

 theory, but the demonstration of the uni- 

 cellular character of the ovum and sperma- 



'■ Newport 's observations rose to a higher plaue 

 than those of the others, for he actually observed 

 in the frog's egg (1) that the first plane of cleav- 

 age is in line with the point on the egg artificially 

 impregnated, (2) that it marks the plane of sym- 

 metry of the embryo, (3) that the head of the 

 young frog is turned towards the same point. 



tozoon had little effect upon the problems of 

 fertilization. The cell theory was still in- 

 complete ; the free formation of the nuclei 

 was still held by competent naturalists, and 

 nothing was known of the phenomena of 

 karyokinesis. The cytological investiga- 

 tions of the next ten years (1874-1884) 

 were destined to lay the foundations of the 

 modern nuclear theory in its broad outlines. 

 The fertilization studies of this period were 

 mainly morphological, and while it is cor- 

 rect to say that they were largely dominated 

 by the growing nuclear theory, it is also 

 strictly true that they contributed in no 

 small measure to its upbuilding. Though 

 the penetration of the spermatozoon into 

 the egg had long been suspected, it was first 

 clearly demonstrated in this time; the or- 

 igin of the egg nucleus by two successive 

 divisions of the germinal vesicle was dis- 

 covered; the origin of the sperm nucleus 

 from the head of the spermatozoon, the 

 sperm aster, the union of the egg nucleus 

 and the sperm nucleus, the relation of 

 these to the first cleavage spindle, the or- 

 igin of the fertilization membrane, the ill 

 effects of polyspermy and the theory of its 

 prevention; and finally the doctrine of the 

 equivalence of the egg and sperm nuclei, 

 and the biparental character of the nuclei 

 of sexually produced organisms, as first 

 laid down by Van Beneden, were products 

 of the period also. No period of cytological 

 research seems to me of greater significance 

 than this. 



There was almost a complete cessation of 

 investigation from 1855-1873, when the 

 dawn of the modern period broke suddenly. 

 In 1873 Biitschli observed in the egg of a 

 nematode the approach and contact of the 

 two structures, which we now know to be 

 the germ-nuclei, immediately preceding the 

 first cleavage of the ovum. But no inter- 

 pretation was presented. In 1874 Auer- 



