Gametogenesis, Fertilization and Parthenogenesis 



181 



location-bearing chromosomes of Drosophila. 

 However, Cooper ('48) has effectively dis- 

 puted the significance of this evidence and 

 concludes that there is, as yet, no necessity 

 for the assumption of long range forces. 



A view of meiosis that has gained both 

 widespread acceptance and controversy is 

 the "precocity theory" of Darlington ('31, 

 '37). According to this view the prophase 

 begins relatively earlier in meiosis than in 

 mitosis, so that the leptotene chromosomes 

 are single rather than split. Synapsis then 

 results from a "universal mitotic affinity of 

 half-chromosomes for one another in pairs." 

 It is further assumed that this relationship 

 is upset by the division of the chromosomes 

 (at pachytene) but that chiasmata resulting 

 from crossing over hold the original pairs 

 together until metaphase, so that normal 

 segregation may occvir. It has, however, been 

 shown by a number of workers (see Cooper 

 '44, '49, for references and convincing evi- 

 dence) that in many species of animals 

 crossing over and the resultant chiasmata are 

 not necessary for metaphase pairing and or- 

 derly segregation. So, that part of Darling- 

 ton's hypothesis cannot be considered to have 

 general validity. Also, there is considerable 

 evidence (Schrader, '44; Mickey, '46, '47) 

 that the leptotene threads are already split, 

 as are the prophase chromosomes in ordinary 

 mitosis. A view that seems to be more con- 

 sistent with the known features of meiosis is 

 the "retardation theory" proposed by Sax 

 and Sax ('35). According to this view the 

 spiral form of the chromonemata of mitotic 

 chromosomes prevents (because a sufficient 

 number of identical loci cannot come in con- 

 tact) intimate association of homologous 

 chromosomes. In the prolonged prophase of 

 meiosis the chromonemata are greatly ex- 

 tended and uncoiled, or have only very loose 

 remnant coils, so that homologous chromo- 

 somes can adhere intimately. Upon the oc- 

 currence of a new split in each chromatid at 

 late pachytene, coiling and separation of 

 homologues begins. This view is consistent 

 with the fact that the chromosomes are con- 

 siderably longer in meiotic prophase than 

 in ordinary mitotic prophase and with such 

 apparently unusual cases as the dipteran 

 giant salivary chromosomes in which the 

 greatly extended polytene homologues re- 

 main permanently synapsed. 



Undoubtedly the analysis of meiosis would 

 be greatly helped by its experimental induc- 

 tion in somatic tissue. It is interesting to 

 note, then, that Huskins ('48) has reported 

 the occurrence of chromosome segregation 



and reduction in onion root tips grown in a 

 solution of sodivim nucleate. However, as 

 Huskins points out, further work will be re- 

 quired to determine the relationship of this 

 "somatic meiosis" to the gonocytic meiosis. 



FERTILIZATION 



One may define fertilization as the series 

 of processes by which the spermatozoon 

 initiates and participates in the development 

 of the egg. As such it includes all steps from 

 the approach of the spermatozoon to the fu- 

 sion of the pronuclei within the egg. In this 

 chapter we shall consider briefly some of 

 the factors that may be operative in various 

 phases of this series of processes. 



APPROACH OF THE SPERMATOZOON 



In most species of animals the meeting of 

 egg and spermatozoon is facilitated by virtue 

 of the fact that the latter is a motile cell. 

 Also, the large numbers that are ordinarily 

 available in both external and internal in- 

 semination contribute to the likelihood of 

 contact being made with the eggs. For ex- 

 ample, Farris ('49) reports that fertile men 

 supply semen samples with a total of 83 

 million or more motile sperm, whereas a 

 lower total active sperm count is coiTelated 

 with infertility. However, the question has 

 been raised by many investigators whether 

 or not the spermatozoa are attracted in some 

 way to the egg when their random move- 

 ments have brought them within a certain 

 distance of the latter. The early investiga- 

 tions concerning chemotaxis has been re- 

 viewed by Morgan ('27), who concluded 

 that there is no critical evidence demonstrat- 

 ing its existence. Such evidence, as well as 

 more recent work along this line (Corn- 

 man, '41; Hartmann, '40; Vasseur and Hag- 

 strom, '46), rests on demonstrating a local 

 accumulation of sperm within tubes or other 

 devices containing eggs or certain materials 

 derived from the eggs. However, it is not 

 readily feasible to distingviish between an 

 attractive influence and a trap action effect 

 such as was described many years ago by 

 Jennings ('06). In ferns and mosses there is 

 good evidence for chemotaxis but it has not, 

 as yet, been adequately demonstrated for ani- 

 mal spermatozoa (cf. Rothschild, '51a, b, '52). 



On the other hand an increase in the mo- 

 tility of spermatozoa under the influence of 

 material emanating from the egg has been 

 noted in some species of animals. This in- 

 crease does not generally occur when the 



