602 FERTILIZATION 



ing a pronucleus. These pinch off as gametes, and the walls between the 

 associated gametocytes break down, allowing the gametes of one to fuse 

 with those of the other. Thus cross-fertilization occurs and the process 

 is isogamous, as there is no differentiation between gametes in this 

 species. 



Calkins and Bowling (1926), working on a species of Monocystis, 

 have confirmed Mulsow's interpretations and have furnished additional 

 critical evidence in support of Mulsow's belief. They found the early 

 progamous divisions with the diploid number of chromosomes (ten) 

 in each daughter plate and also the final progamous divisions with the 

 haploid number (five) in each daughter plate. 



Naville (1927a) has shown that in three types of Monocyst/s reduc- 

 tion is gametic. In types "A" and "B" early divisions of the pseudocon- 

 jugants show eight chromosomes and type "C" shows four as the diploid 

 number. Anaphases of the last two divisions preceding gamete forma- 

 tion show four chromosomes going to each pole in types A and B, and 

 two in type C. The next to the last division, therefore, is the reduction 

 division. The first amphinuclear division is not reductional, and the 

 sporoblast is diploid during its subsequent development. 



Naville (1927b) also showed that in Urospora lagidis reduction of 

 chromosome number from a diploid four to a haploid two occurs in the 

 formation of its anisogamous gametes. A noteworthy occurrence here 

 is that synaptic conjugation of chromosomes takes place in the synkaryon. 

 The subsequent division of the synkaryon is equational, but the phe- 

 nomenon serves to illustrate the possibility, in other forms, of an ex- 

 tremely precocious synapsis being prolonged throughout the life cycle 

 until the next sexual stage appears, when the two members of the syn- 

 aptic pairs would separate in a progamic division. Such a condition, if 

 it exists, would explain in terms of gametic reduction the few known 

 examples of zygotic reduction. This hypothesis seems worth investigat- 

 ing. Valkanov (1935) found pairing of chromosomes (synapsis) and 

 condensation of chromosomes (diakinesis) into rings and crosses, in 

 the zygotes of Monocystella arndti; and he believes that reduction oc- 

 curs in the first zygotic division (see p. 6l3 below), although he was 

 not able to follow the subsequent behavior of the chromosomes. His 

 figures show eleven pairs in the zygote and eleven single chromosomes 

 in the early divisions of the pseudoconjugants. These numbers indicate 



