292 SUMMARY OF CURRENT RESEARCHES RELATING TO 



becomes normal ; probably through longitudinal splitting without a 

 corresponding division of the cyptoplasm. In all parthenogenetic e«i:*, 

 with possibly the one exception of Nematus (according to Doncaster), the 

 number of chromosomes in the first cleavage nucleus becomes in some 

 way equal to that in the somatic cells. It is not impossible that in 

 Sonatas there is no reduction in spermatogenesis, which would lead 

 to the same thing. Petrunkevitch's attempt to show that the polar cells, 

 after a conjugation similar to that of the pronuclei, give rise to the 

 primordia of the male gonads, requires confirmation. 



Experiment must supplement the observation of normal partheno- 

 genesis. Two methods are possible, of which one is cross-breeding and 

 in-breeding, e.g. in bees, and the other artificial parthenogenesis and 

 merogony. In merogony there is a reduced number of chromosomes 

 and the presence of the active sperm-centrosome. In artificial partheno- 

 genesis, there is a stimulation to new life of the egg-centrosome. It 

 may be that there are de novo formations of centrosomes, as the research 

 of Yatsu has again emphasised ; even then, it is probable that the 

 divisions are due to the egg-centrosome. 



According as the stimulus is appplied before or after the second 

 maturation division, we get development with the normal number of 

 chromosomes, or with the reduced number. Both parthenogenetic 

 development with the reduced number of chromosomes and merogonic 

 development show abnormalities, which increase in inverse ratio to the 

 number of chromosomes left in the egg ; all this goes to show that 

 neither merogony nor artificial parthenogenesis with a reduced number 

 of chromosomes can be regarded as equivalent to natural parthenogenesis. 

 So Petrunkevitch distinguishes artificial, pathological, uniparental 

 development from artificial true parthenogenesis (with the normal number 

 of chromosomes). Experiments must be made by applying the stimulus 

 at the different moments of maturation. 



Maturation and Fertilisation of Haminea solitaria (Say).*— TV. M, 

 Smallwood gives the results of his embryological investigations upon this 

 mollusc. In the first maturation the chromosomes divide transversely, 

 but in the second maturation it is difficult to ascertain whether the 

 division is transverse or longitudinal. A quantitative, but not a 

 numerical, reduction is accomplished by each of these two divisions. 

 Qualitative reduction cannot be demonstrated. In fertilisation, the sperm 

 head may penetrate the ovum at any point of its surface. A middle 

 piece .could not be distinguished. Accessory asters appear in the ana- 

 phase of the second maturation, which are temporary and independent 

 of the sperm head. The chromatin in both male and female pronuclei 

 passes through a metamorphosis before the cleavage asters arise. Of the 

 two cleavage asters one is associated in origin with each pronucleus. 

 The centrosome at the close of each cleavage becomes indistinguishable 

 from the microsomes in the cytoplasm, and it is therefore impossible to 

 affirm that it divides in preparation for the following cleavage. 



Individuality of the Chromosomes.! — TV. J. Baumgartner commu- 

 nicates some observations on the germ-cells of crickets, which furnish 



* Bull. Mus. Comp. Zool. Harvard, xlv. (1904) pp. 259-31S (13 pis.). 

 t Biol. Bulletin, viiL (1904) pp. 1-23 (3 pis.). 



