296 EMBRYOLOGY 



and also belong to the most important discoveries of modern biology. 

 Here also the discoveries for which we must thank, in the first place, 

 zoologists and embryologists, have reacted favorably upon the in- 

 vestigations of botanists. 



The older zoologists, Fritz Miiller, Loven, and others, had already 

 noticed that from the egg-cells of the most distant classes of ani- 

 mals two minute spheres of protoplasm were thrown off, a short 

 time before or during fertilization (Diagram in, Figs. 3, 4, 5, pz l , 

 pz 2 p). These were called the polar bodies, because at the place of 

 their extrusion from the ovum the first plane of segmentation began. 

 Their importance remained an enigma. Many scientists believed 

 that they constituted an excretion by the extrusion of which the egg 

 purified itself of useless substances, before the beginning of its 

 further development. Then Biitschli observed that the nucleus of 

 the unfertilized egg is concerned in the formation of the polar bodies, 

 that it projects from the surface of the yolk in the form of a nuclear 

 spindle, which, as he believed, is then extruded in the form of the 

 polar bodies. This was a great advance, but combined with an error 

 with regard to the entire meaning of the process which soon after 

 was rightly determined by Giard and myself. For more accurate 

 investigation showed us that the polar bodies were not formed by 

 extrusion, but by two true divisions which followed immediately 

 upon one another, and that in the second division half of the spindle 

 and of the chromosomes remained behind in the egg, and here became 

 the nucleus of the mature egg. The process only differed from an 

 ordinary cell-division in that the parts were so unequal in size. 

 The polar bodies should, therefore, better be denoted polar cells. 



For what reason and to what end, we may ask, are these two 

 insignificant polar cells formed with such great regularity in the 

 entire animal kingdom? On this also light was soon thrown by the 

 accurate study of an extremely favorable object for investigation, 

 the egg of the horse roundworm, Ascaris megalocephala, which has 

 been as productive of results in the study of the process of fertiliza- 

 tion as the egg of the Echinodermata. Its invaluable advantage 

 consists in the fact that it gives us a deeper insight into the relation 

 of that substance which plays the most important role in the divis- 

 ion of the nucleus, namely, the chromatin. 



Of the chromatin we know by investigations which are among 

 the most brilliant of the histological advances of the last decennium 

 (see Diagram u, showing nuclear and cell division) that the chro- 

 matin at the beginning of the nuclear division is changed into a 

 long convoluted chromatin thread, and that this in the second phase 

 (Fig. 2) breaks up by cross-segmentation into a very definite number 

 of segments or chromosomes (ch) which arrange themselves in the 

 middle of the nuclear spindle (Fig. 3, sp) to a symmetrical figure, 



