298 EMBRYOLOGY 



the mother star. Each chromosome then begins to split longi- 

 tudinally into two identical parts, the two daughter chromosomes 

 (Figs. 3, 4, 5, ch). We are justified in seeing in this the true task of 

 the complicated nuclear division, as the two halves now move away 

 from one another toward the opposite ends of the nuclear spindle 

 (Fig. 5, ch l and ch?} and form the two daughter stars, which after 

 the division of the cell in two parts form in each the basis of a 

 daughter nucleus. These promptly return to the spherical form. 



Extended comparative observations in the most widely separated 

 classes of animals have demonstrated a definite numerical law in the 

 chromosomes. It states: In all cells of an animal or plant species 

 the same number of chromosomes always arise during a division 

 of the nucleus. In one species four, in another twelve or sixteen or 

 twenty-four, etc. The number of chromosomes is four only in one 

 variety of Ascaris. For this reason, and because the few chromo- 

 somes are at the same time of very considerable size, the eggs of the 

 horse roundworm are of great advantage for studies in the question 

 which now concerns us. 



These remarks with regard to the phenomena of nuclear division 

 must first be made, in order to understand the progress which has 

 been brought about by the study of Ascaris eggs in the remarkable 

 investigations of van Beneden, which immediately followed the 

 excellent work of Boveri. 



Two fundamental facts were discovered concerning the behavior 

 of the chromatin in the ^.scan's egg (Diagram in). One of these 

 facts concerns the process of fertilization. Egg- and sperm-nuclei 

 (Fig. 5, eik and sk) remain, in the egg of Ascaris, separated from 

 one another for several days, and prepare themselves separately 

 for the formation of the first karyokinetic spindle. From the chro- 

 matin network, chromosomes arise in the way described above, 

 two in the egg-nucleus (Fig. 5, wch), two in the sperm-nucleus (mch). 

 We can thus easily follow their fortune in the further stages of di- 

 vision, and determine that of the four chromosomes of the nuclear 

 spindle, two arise from the egg-nucleus, two from the sperm-nucleus. 

 When the chromosomes split longitudinally, in the stage of the 

 mother star, we see their products, the daughter chromosomes, sepa- 

 rate from each other, in the way described above (Fig. 7, wch and 

 mch), to form the daughter stars, and finally enter into the forma- 

 tion of the daughter nuclei of the two new cells. In this case incon- 

 trovertible proof has been brought that in the first division of the 

 fertilized egg an equal amount of chromatin from the egg-nucleus 

 and the sperm-nucleus is brought to each of the daughter nuclei. 



This process apparently repeats itself in every later division, 

 so that finally the nucleus of every tissue cell is composed of equal 

 amounts of chromatin of maternal and paternal origin, which has 



