4 GENERAL INTRODUCTION. 



These facts justify the conclusion that the nuclei of the two germ-cells are in a morphological sense precisely 

 equivalent, and they lend strong support to Hertwig's identification of the nucleus as the bearer of hereditary qualities. 

 The precise equivalence of the chromosomes contributed by the two sexes is a physical correlative of the fact that the 

 two sexes play, on the whole, equal parts in hereditary transmission, and it seems to show that the chromosomal substance, 

 the chromatin, is to be regarded as the physical basis of inheritance. Now, chromatin is known to be closely similar to, 

 if not identical with, a substance known as nuclciu (Cg Hj.j N,, P3 O.w, according to Miescher), which analysis shows to be 



PB 



PB 



Fi.:. III. 

 Fig. III. — History of the germ-nuclei in the KYi^i'i.iX-wmm Ascaris megalocephaUi (after Boveri), highly magnified. 



A. Egg immediately after the formation of the second polar body, PB; E, the egg-nucleus, consisting of two branching chromosomes; S, the sperm-nucleus derived from the head 



of a spermatozoon that has entered the egg. 



B. Following stage, in which the egg-nucleus (E) and sperm-nucleus (S) have assumed the same size and structure. 



C. Later stage, in which the substance of each germ-nucleus has become transformed into two chromosomes, — i.e., each sex has contributed two of these bodies, exactly similar in 



form and size. On either side, at A, is an attraction-sphere or archoplasm-sphere, each containing a single centrosome. These were originally supposed by \ an Beneden to be 

 derived one from each sex. Boveri showed that the two arise by the division of one sphere, which is developed under the influence of a centrosome, derived from the spermato- 

 zoon alone. 



D. The karyokinetic figure forming for the first cleavage. The archoplasm-spheres have given rise to two asters (A) between which a spindle (S) is forming. The chromosomes 



(C) are splitting lengthsvisc, the two halves being destined respectively for the two daughter-cells. 



a tolerably definite chemical composed of nucleic acid (a complex organic acid rich in phosphorus) and albumin. And 

 thus we reach the remarkable conclusion that inheritance may, perhaps, be effected by the physical transmission of a 

 particular chemical compound from parent to offspring. 



Structure of the Resting-cell. — It is now necessary to consider briefly the structure of the resting-cell as an 

 introduction to the subject of cell-division, which, in turn, leads us to another aspect of fertilization. 



In the resting-cell (Text-fig. V. A) the protoplasm of the cell-body, or cytoplasm, consists of a network or cyto-rcticiilnm 



