72 EMBRYOLOGY. 



first time a scheme of nuclear division and cell-division, which has since 

 proved to be correct in all essentials, even though it has undergone important 

 improvements and additions at the hands of FOL, FLEMMING, VAN BENEDEN, 

 and KABL. 



FOL published an extended monographic investigation of the process of 

 cleavage, which he had observed in many invertebrated animals. FLEMMING, 

 starting with nuclear division in tissue-cells, distinguished with great acumen 

 the non-chromatic and the chromatic parts of the nuclear figure, the non- 

 stainable nuclear spindle- fibres, and the stainable nuclear filaments and loops,, 

 which are located upon the surface of the former. He made the interesting 

 discovery concerning the latter, that they become split lengthwise. Ligbt 

 was soon thrown upon this peculiar phenomenon, when HEUSER, VAN BENEDEN, 

 and BABL, independently of each other, discovered that the halves of the split 

 filaments moved apart toward the poles of the nucleus, and furnished the 

 fundament for the daughter-nuclei. VAN BENEDEN at the same time made 

 the additional and important observation on the egg of Ascaris megalocephala, 

 that of the four chromatic loops, which are constantly to be observed in the 

 case of the cleavage-nucleus, two are derived from the chromatic substance 

 of the spermatic nucleus, the other two from the chromatic substance of the 

 egg-nucleus ; and that, in consequence of the longitudinal splitting, each 

 daughter-nucleus receives at the time of division two male and two female 

 nuclear loops. In addition there have appeared many other recent works 

 of value on the process of cleavage by NUSSBAUM, EABL, CARNOY, BOVERI, 

 PLATNER, and others. 



Within the last few years PFLtiGER has endeavored to prove by interesting 

 experiments that gravitation exercises a determining influence on the position 

 of the planes of cleavage. BORX, Roux, and the author, on the contrary, 

 thought they were able to explain division from the organisation of the egg- 

 cell itself. In the author's article, " Welchen Einfiuss ubt die Schwerkraft 

 auf die Theilung der Zellen ? " he recognised the causes which determine the 

 various directions of the planes of division, (1) in the distribution of the 

 lighter egg-plasm and the heavier deutoplasm, and (2) in the influence which 

 the spatial arrangement of the egg-plasm exercises on the position of the 

 nuclear spindle, and that which the position of the latter exercises upon the 

 direction of the plane of cleavage. 



SUMMARY. 



1. In the process of cleavage the internal and the external pheno- 

 mena of segmentation are to be distinguished from each other. 



2. The internal phenomena of cleavage find expression in changes 

 (a) of the nucleus, (6) of the protoplasm. 



3. The nucleus while in the process of division consists of a non- 

 chromatic and a chromatic nuclear figure. The non-chromatic figure 

 is a spindle composed of numerous fibres. The chromatic figure is 

 formed of bent, Y-shaped nuclear filaments (chromosomes), which lie 

 upon the surface of the middle of the spindle. At the two ends of 

 the spindle there is found a special polar corpuscle [centrosome]. 



