332 MR. L. T. HOGBBN ON CERTAIN NUCLEAR PHENOMENA 



If this interpretation prove to be correct, the separation of the polar 

 chromosome groups in Neuroterus is the completion of a mitotic process that 

 begins in the oocyte before it is laid. The maturation spindle in the late 

 oocyte is a precocious and interrupted metaphase, preceded by an end-to- 

 end pairing of univalents, and only differing from the normal type in 

 consequence of its previous history and in the absence of asters or centro- 

 somes. According to Doncaster, however, there are neither asters nor 

 centrosomes in the polar and segmentation mitoses of the Saw-flies. In this 

 case the solid and top-shaped nuclei of the full-grown egg in Neuroterus are 

 not nuclei in the strict sense of the word. The only account, as far as I am 

 aware, of a comparable prolongation of the maturation process is that of 

 Histriobdella, where Shearer describes a protracted prophase lasting for a 

 week while the cytoplasm of the egg was still in its growing phase. 



It follows that in those Hymenoptera where similar maturation spindles 

 have been described in the late oocyte, irregular methods for the extrusion 

 of polar nuclei should prevail also. This appears to be the case in Martin's 

 account of Ac/ehiaspis, though details are lacking. Gratenby has described 

 the formation of polar bodies in the Chalcid Trichogramma by what he terms 

 " amitosis." It is, of course, of immense importance from the standpoint of 

 the chromosome hypothesis to know whether amitosis occurs in germ nuclei ; 

 and if it is accepted that the nucleus of the full-grown egg in the parasitic 

 Hymenoptera is in reality a condensed equatorial plate, an apparent con- 

 striction, simulating amitosis, does not imply an unequal distribution of 

 chromatic elements, seeing that these are definitely orientated lengthwise in 

 the complex. I have no hesitation in saying that the formation of polar 

 bodies in Trichogramma cannot be justifiably contended to detract from the 

 accepted views as regards the persistent individuality of the chromosomes. 



Mention has been made of Henking's description of the maturation of 

 the egg in Rhodkes, the solid nucleus of which is reminiscent of that 

 of Neuroterus, the separation of the polar chromosomes being also similar. 

 A comparison of Schleip's figures of the chromosomes in the late oocyte with 

 the same stage in Andricus, as shown by Hegner's plates, suggests very 

 forcibly that they are in reality doubles and not univalent as Schleip 

 believed*. I must, however, refrain from further comment on this head, 

 seeing that I hope shortly to publish a full account of the chromosome cycle 

 in this form. 



I have to thank Dr. Doncaster for providing me with material for these 

 observations, as well as for advice. 

 June 1919. 



* I have since found that such is actually the case. 



