26 * EMBRYOLOGY. 



division ; and has consequently made the three groups of alecithal, telolecithal, 

 and centrolecithal eggs, for which I have selected the designation eggs with 

 little or uniformly distributed yolk, eggs with polar, and eggs with central 

 yolk. 



In recent times investigation has been directed to the finer structure of the 

 germinative vesicle, in which KLEINENBEKG (1872) was the first to observe a 

 special protoplasmic nuclear trestle (Kerngerilsf) or nuclear network, which since 

 then has been shown by numerous researches to be a constant structure. In 

 the case of the germinative dot I have myself designated two chemically and 

 morphologically distinguishable substances as nuclein and paranuclein, the 

 investigations concerning the importance and the role of which in the develop- 

 ment of the egg are not yet concluded. 



The history of the spermatozoa begins with the year 1677. A student in 

 Leyden, HAMM, in the microscopic examination of semen, saw the briskly 

 moving bodies, and communicated his observation to his teacher, the celebrated 

 microscopist LEEUWENHOECK:, who instituted more accurate investigations, 

 and published them in several papers, which soon attracted general attention. 

 The sensation caused was all the greater becausa LEEUWENHOECK declared the 

 seminal filaments to be the preexisting germs of animals, and maintained that 

 at fertilisation they penetrated into the egg-cell and grew up in it. Thus 

 arose the school of animalculists. 



After the refutation of the preformation theory, it was thought that no 

 importance was to be ascribed to the seminal filaments in fertilisation, it 

 being held that it was the seminal fluid that fertilised. Even during the first 

 four decennia of the present century, the seminal filaments were almost 

 universally held to be independent parasitic creatures (spermatozoa) com- 

 parable with the Infusoria. Even in JOH. MULLER'S " Physiology " (1833-40) 

 occurs this statement : " Whether the semen-animalcules are parasitic animals, 

 or animated elements of the animals in which they occur, cannot for the 

 present be answered with certainty." 



The settlement of the question was accomplished by comparative histological 

 investigations of the semen in the animal kingdom, and by physiological 

 experiment. 



In two essays " Beitriige zur Kenntniss der Geschlechtsverhaltnisse und 

 der Samenfliissigkeit wirbelloser Thiere," and "Bildung der Samenfaden in 

 Blaschen " KOLLIKEB showed that in many animals, e.g., in the Polyps, the 

 semen consists of filaments only, the fluid being entirely absent ; and that in 

 addition the filaments are developed in cells, and consequently are themselves 

 elementary parts of animals. KEICHERT discovered the same to be true in 

 Nematodes. By means of physiological experiment it was recognised that 

 seminal fluid with immature and motionless filaments, and likewise mature but 

 filtered semen, did not fertilise. This was decisive for the view that the 

 seminal filaments are the active part in fertilisation, and that the fluid, which 

 is added thereto in the case of the higher animals under complicated sexual 

 conditions, "can be regarded only as a menstruum for the seminal bodies 

 which is of subordinate physiological significance." 



Since then our knowledge (1) of the finer structure, and (2) of the develop- 

 ment of the seminal filaments, has made further advances. So far as regards 

 the first point, we have learned, especially through the works of LA VALETTE 

 and SCHWEIGGER-SEIDEL, to distinguish between head, middle piece, and 



