Ill 



PARTHENOGENESIS 89 



discussion, syndesis takes place in the usual manner, and that the 

 homologous chromosomes become completely, instead of partially, dis- 

 sociated in the diplotene stage. Instead of pairing again to form the 

 bivalents of the meiotic mctaphase, each chromosome behaves from now 

 onwards as it would in a somatic mitosis. 



Strasburger (1907, 1909) was the first to elaborate this view in the 

 case of plants, and has in fact described the dissociation of the haploid 

 bivalents into diploid univalents in the parthenogenetic (" apogamous ") 

 development of the macrospore of the cryptogam Marsilia driimmondii. 



For animals, however, it still remains nothing more than a conjecture. 



Although in the parthenogenetic species considered so far the meiotic 

 prophases are scarcely distinguishable from those of sexual forms (except 

 for the diploid chromosome number and the occurrence of only one instead 

 of two maturation divisions), there exist other forms where the meiotic 

 prophase stages are markedly different in alhed parthenogenetic and 

 sexual species. This again was first described in plants (Urticaceae) 

 by Strasburger (1910). 



In animals. Fries (1910) found in the crustacean Branchipus (Fig. 

 42), which reproduces sexually, all the ordinary phases of meiosis, namely, 

 a leptotene stage followed by synizesis, during which the leptotcne 

 ^ threads arrange themselves in parallel pairs ; these apparently fuse to 

 ' form pachytene bands which on the dissolution of synizesis are found 

 in the haploid number. In the nearly alhed but parthenogenetic Artemia 

 salina, however, there is no synizesis, nor parallelization of leptotene 

 threads, nor fusion of these to form pachytene bands. On the contrary, 

 each leptotene thread condenses into a single chromosome, and conse- 

 quently these are present in diploid number. Again, however, we find 

 the definitive chromosomes— in the one case bivalents, and in the other 

 split univalents— surprisingly alike in appearance in the two genera. 



Morgan (1915 a) also found in the Aphids Phyllaphis and Phylloxera 

 that in the ovaries of sexual females the meiotic prophases include a 

 synizesis, into which the full number of chromosomes (6) enter, and from 

 which three bivalents emerge. In the ovaries of the parthenogenetic 

 members of the same species, the six chromosomes of the early meiotic 

 prophase contract continuously into six univalents without ever being 

 condensed in a synizetic contraction. 



A very few cases of obhgatory parthenogenesis are known in wliich 

 two meiotic divisions occur as in sexual reproduction, but without 

 reduction of the chromosome number. The mature cgQ is therefore in 

 the same condition as in the ordinary cases of obligatory parthenogenesis 

 with only one meiotic division. This very puzzling phenomenon was 

 first (with the exception of the special case of Artemia) described by 



