SYNGAMY 243 



modes of behavior : (a) the chromatic sperm nucleus may be added to the 

 comparatively achromatic egg nucleus, the two forming a common 

 reticulum from which the chromosomes later condense; (6) the sperm 

 nucleus may become achromatic before uniting with the egg nucleus, the 

 achromatic fusion nucleus forming a reticulum from which the chromo- 

 somes develop; (c) the sperm nucleus may condense its chromosomes 

 before uniting with the egg nucleus, no common reticulum being formed 

 and the maternal and paternal chromosomes remaining separate through- 

 out their development. 



In the first cleavage mitosis, whether the gamete nuclei have formed 

 a common reticulum or not, each chromosome undergoes a longitudinal 

 division, the halves passing to the two daughter nuclei. This process is 

 repeated in every subsequent somatic mitosis, so that every somatic 

 nucleus has descendants of every parental chromosome originally present 

 in the fertilized egg. In most animals the first cleavage mitosis is fol- 

 lowed by the division of the fertilized egg into two cells (blastomeres), 

 but in some cases there are several free nuclear divisions before any 

 cleavage into cells occurs. In some instances it has been shown that the 

 plane of the first cleavage is determined by the chance point of entrance of 

 the spermatozoon, and that the bilaterality of the embryo bears a direct 

 relation to this cleavage. ^^ In other animals, on the contrary, the plane 

 of the first cleavage does not coincide with the plane of bilateral symme- 

 try; moreover, some eggs may show such symmetry before fertilization 

 (e.g., many insects). 



The more or less independent formation of the maternal and paternal 

 chromosomes without an intimate nuclear fusion before the first cleavage 

 mitosis is known as gonomery and has been observed in both animals and 

 plants (Fig. 143). It occurs in Drosophila (Huettner, 1924). Further- 

 more, in several organisms it has been found that the two parental groups 

 of chromosomes can be distinguished not only in the first embryonal 

 mitosis but also in several divisions thereafter. ^^ This is especially 

 evident in hybrids. There is reason to believe that the chromosomes 

 brought together by the two gametes, although they are intermingled in 

 the nuclei of the new individual, never actually fuse in such a way as to 

 lose their essential organization. They are joined with one another in the 

 reticulum and undergo occasional transfers and exchanges of parts, as will 

 be shown in subsequent chapters; but this does not involve the complete 

 mixing of their substance which was once thought to occur. The evident 

 fact that certain nuclear elements maintain their identity through suc- 



32 Wilson and Mathews (1895) on the sea urchin; Just (1912) on Nereis. See 

 Morgan (19246) and Wilson (1925; p. 1012). The plane of cleavage is associated 

 with gravity by Giglio-Tos (1926). 



" Riickert (1895) and Haecker (1895c) on Cyclops; Conklin (1901a) on Crepidula; 

 B. G. Smith (1919) on Crypiobranchus. 



