Gametogenesis, Fertilization and Parthenogenesis 



201 



urodeles, that have been undertaken prin- 

 cipally by Fankhauser (see '48 for references 

 and review J, have contributed important 

 information concerning the factors that in- 

 lluence pronuclear behavior. In urodeles all 

 of the sperm that enter undergo the normal 

 pronuclear changes during the first few 

 hours. However, after one of the sperm pro- 

 nuclei has fused with the egg pronucleus 

 the remainder begin to degenerate, those 

 nearest the fusion nucleus regressing first 

 (Fig. 67 A). If the egg is constricted after 

 fertilization so that one fragment contains a 

 sperm pronucleus (see Fig. 67 B), this can 

 proceed to form a spindle, and the fragment 

 can undergo cleavage and normal develop- 

 ment, as earlier work had also shown. How- 

 ever, if the constriction leaves a rather broad 

 connecting neck between the two parts, the 

 development of the accessory sperm pro- 

 nucleus is inhibited. These and other ob- 

 servations establish that some inhibitory 

 influence emanates from the fusion nucleus. 

 This covild involve either the production of 

 an inhibitory agent or the removal of some 

 agent essential for the division of the acces- 

 sory cytaster and the nuclear transformations. 

 It could also involve mechanical factors, 

 since the enlarging amphiaster displaces the 

 accessory sperm pronuclei towards regions 

 of the egg where conditions may not be 

 proper for their further development. 



ARTIFICIAL PARTHENOGENESIS 



Relatively little work in this field has 

 appeared since the last general review of the 

 subject (Tyler, '41). Detailed accounts of 

 earlier work are available in texts by Loeb 

 ('13), Lillie ('19), Morgan ('27) and Dalcq 

 ('28b). Summaries of methods employed in 

 the artificial activation of eggs of various ani- 

 mals have been given by Harvey ('10) and 

 Ruimstrom ('28). In this section a brief 

 summary is presented of some of the features 

 of artificial parthenogenesis previously dis- 

 cussed (Tyler, '41), along with a short ac- 

 coimt of recent views on the problem of 

 activation. 



SUMMARY OF SOME GENERAL 



FEATURES OF ARTIFICIAL 



PARTHENOGENESIS 



Advanced Stages. It has been established 

 that artificially activated eggs of many ani- 

 mals (sea lu-chins, starfish, moths, fish, frogs, 

 rabbits) can be reared to the adult condition 

 and sexual maturity. Normal early develop- 



ment is obtainable in practically all the 

 major groups of animals and it is clear that 

 when normal embryos are produced, obtain- 

 ing the adults is mainly a matter of appro- 

 priate culture methods. 



Frequency of Normal Development. There 

 have been only occasional reports of experi- 

 ments in which all of the eggs respond with 

 normal development to a particular activat- 

 ing treatment. In general the percentage of 

 normal development is quite low even when 

 a particular treatment initiates development 

 in all of the eggs in a manner indistinguish- 

 able from that induced by the sperm. This 

 is attributable to various factors, such as 

 irregularities in distribution of chromosomes, 

 haploidy, lack of a proper division mecha- 

 nism (normally supplied by the central body 

 of the sperm) and, in some instances, to fail- 

 ure to establish a plane of bilateral symmetry 

 (determined in some species by the entrance 

 point of the sperm). 



Origin of Cleavage Amphiaster. In sea 

 urchins cleavage amphiasters are evidently 

 derived from cytasters that are induced by the 

 artificial treatment. In various annelids and 

 mollusks cytasters are not in general formed. 

 Here the amphiaster arises as a result of 

 suppressed polar body divisions and con- 

 tinued cleavage involves a renewal of the 

 capacity for division on the part of the 

 central bodies of the egg and polar bodies. 

 In polar body suppression, (a) the presump- 

 tive polar body spindle may be converted 

 into a cleavage spindle, (b) submerged polar 

 divisions (without cytoplasmic division) may 

 occur followed by fusion of nuclei and as- 

 sociation of central bodies. In the latter type 

 of suppression an amphiaster, triaster or 

 tetraster may form depending upon whether 

 only the first, the first and one of the second, 

 or the first and both second polar divisions 

 are submerged, and the first cleavage then 

 correspondingly results in two, three or four 

 cells (see Fig. 68). 



Activation of Non-nucleate Egg Fragments. 

 The formation of cytasters capable of multi- 

 plication can be induced in non-nucleate 

 fragments of sea urchin eggs. Such fragments 

 may undergo repeated cleavages. This im- 

 portant work of E. B. Harvey acquires further 

 interest in that it may indicate the possibility 

 of de noT'o formation of self-duplicating 

 bodies in the cytoplasm. 



Chromosome Numbers. Artificially acti- 

 vated eggs do not invariably develop as 

 haploids. In fact those that have been reared 

 to advanced stages have the diploid chromo- 

 some number, whereas the haploids generally 



