GENERATION AND INHERITANCE 



Fig.l. Fig.Z. 



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DIAGRAM III. The process of fertilization, the formation of polar cells and the first 

 division of an egg of Ascaris megalocephala bivalens. 



FIG. 1. The egg at the moment of fertilization. It shows still a spherical nucleus 

 (kb) in which the chromatic substance is arranged in two groups of four (tetrads, 

 ch). The spermatozoon, shaped like a tailed sphere, has pressed halfway into the 

 egg. Its nucleus (k) is composed of two chromosomes. 



FIG. 2. From the spherical nucleus a spindle with two tetrads has arisen (ch). 

 The spermatozoon (s) has pressed into the middle of the egg. 



FIG. 3. At the animal pole of the egg, where the spindle lay in Fig. 2, the first 

 polar cell (pzl) has been formed by budding. It receives from each tetrad of the 

 spindle two chromosomes connected in pairs (a dyad), while the other two chro- 

 mosome pairs (ch) remain behind in the egg with the half spindle (sp). The 

 spermatozoon (sk) begins to dissolve, except the nucleus, which begins to become 

 spherical. 



FIG. 4. In the same way as the first the second polar cell is formed by budding 

 (/>2 2 ). From each of the pairs of chromosomes (Fig. 3, ch) of the previous stage, 

 a chromosome comes to live in the second polar cell, while the other remains behind 

 in the egg and forms the egg-nucleus (eik), which then contains two chromo- 

 somes, as does the spermatazoon (sk). 



FIG. 5. Egg- and sperm-nucleus approach each other until they touch, but do 

 not unite. In order to distinguish their chromosomes those of the egg-nucleus are 

 drawn as a white circle (wch), those of the sperm -nucleus as a black circle (inch), 

 as was done in the previous Figs. 1 to 4. 



FIG. 6. Egg- and sperm-nuclei have together formed a spindle of whose four 

 chromosomes half (wch) arise from the egg-nucleus, the other half (mch) from the 

 sperm-nucleus. The polar cells, as m Figs. 7 and 8, have been omitted. 



