GENERAL INTRODUCTION. 3 



The Chromosomes. — The analysis was pushed a step farther in the year 1883, by Edouard van Beneden/ whose 

 discoveries, originally made in the case of the thread-worm Ascaris, were confirmed and extended to many other animals 

 and to plants by Boveri, Strasburger, and others. These discoveries related to the internal structure of the nuclei them- 

 selves. In all cases the sperm-nucleus is at first very much smaller than the egg-nucleus, so that at first sight a marked 

 inequality seems to exist between the two sexes in this respect (cf. Text-figs. XI., XII.). Van Beneden, however, 

 determined the remarkable fact that during fertilization the inequality totally disappears and the two nuclei finally exhibit 

 a precise morphological similarity, as follows. Before or during their union, each of them is transformed into a definite 

 numlDcr of rod-like bodies, known as chromosomes (Waldeyer), which arc of the same form, size, and mimber in the two 

 sexes. Both their form and their number differ in different species, but there is strong reason to believe that they are 



D 



Fig. II. 



Fig. II. — The spermatozoon and its entrance into the egg. {Toxopneustes. From life.) 



A. The spermatozoon, enlarged 3500 diameters. N, nucleus; M, middle-piece; T, tail. 



B. First contact with the naked egg (about 1200 diameters). 



C Formation of the entrance-cone and of the vitelline membrane (V). 



D. The head (nucleus and middle-piece) has entered the egg. The tail is left outside the vitelline membrane. 



E, F, G. Later stages, showing the changes of the entrance-cone. In the last figure (three minutes after the first contact) the sperm-aster has become visible within the egg at S. 



always constant in the same species throughout the animal kingdom. Thus, in Ascaris megalocephala, each germ-nucleus 

 (egg-nucleus and sperm-nucleus) gives rise in one variety of the worm to one chromosome, in another variety to two 

 (Text-fig. III.). In three other species of thread-worms, Coronilla, Spiroptera, and Filaroidcs, the numbers are respectively 

 4, 6, and 8 chromosomes from each sex (Carnoy). The sea-urchin Echinus, has 9; the worm Sagitta, likewise 9; the 

 medusa Tiara, 14; and the mollusk Pterotrachca, 16 (Boveri). (Text-fig. IV.) Toxopneitstcs has either 18 or 19 (probably 

 the latter), from each sex. In the trout the number is 12 (Bohm); in the torpedo, approximately 18; and still larger 

 numbers occur in some of the higher animals, few of which, however, have thus far been accurately determined. Precisely 

 parallel results have been obtained among the plants. In the lily each germ-nucleus gives rise to 12 chromosomes 

 (Guignard); in the onion to 8 (Strasburger); in Pallavicina (one of the HepaticEe), the number is only \: 



1 Recherches sur la maturation de I'oeuf. la fecondation et la division cellulaire. Gand. 1883 ; also. Arch, de Biol., 1884. 



- It has recently been shown that the number of cliromosomes may vary slightly from the normal, but this is very exceptional. In the snail Arion. the sperm- 

 nucleus appears to give rise to only two chromosomes, while the egg-nucleus produces a m.uch larger number (Platner) ; this exception is, however, probably only 

 apparent. 



