172 



of the first cleavage plane of the centrifuged 

 egg so that the first two blastomeres have 

 essentially the same cytoplasmic constitution 

 with regard to factors inducing or prevent- 

 ing diminution. The next division, then, is 

 differential with regard to these factors in 

 each blastomere. 



More recently King and Beams ('38) have 

 subjected Ascaris eggs to high speed (150,000 

 g.) centrifugation whereby cytoplasmic di- 

 vision was suppressed while nuclear division 

 continued. They noted that, in such eggs, 

 diminution usually occurs in all of the nuclei 



AB 

 (ECTODERM) 



(ECTODERM) 



Fig. 47. Diagram illustrating location, in the un- 

 cleaved Ascaris egg, of the material for the primor- 

 dial germ cell (P4) (after von Ubisch, '43). 



at the second or the third mitosis. This led 

 them to interpret diminution on the basis of 

 a "diminisher" substance Z>, that is produced 

 from some cytoplasmic constituent. An, 

 which is concentrated in the animal region 

 of the egg and grades off to zero at the 

 vegetal pole. Normally, D reaches sufficient 

 concentration to cause diminution in the 

 polar cell or cells at the second or third 

 cleavage, and in the somatic cells derived 

 from the stem cells at succeeding divisions. 

 In the absence of cell boundaries, in the 

 centrifuged egg, D is assumed to be free to 

 diffuse and, upon reaching sufficient concen- 

 tration, causes diminution to occur in all of 

 the nuclei. This view has been criticized by 

 von Ubisch ('43), who points out that the re- 

 gion of the uncleaved egg that is to be in- 

 corporated into the primordial germ cell 

 (P4) does not comprise the most vegetal ma- 

 terial but is located about halfway between 

 the equator and the vegetal pole (see Fig. 

 47). This fact was evident in the original 

 cell-lineage studies of Boveri (1899) but has 

 been overlooked in most accounts of the 



Embryogenesis: Preparatory Phases 



work on Ascaris. However, it seems to the 

 present author that the essential feature of 

 the interpretation of King and Beams re- 

 mains valid. That feature is the localized 

 production of some substance (either induc- 

 ing or preventing diminution) during early 

 cleavage. It seems simplest to regard this in 

 terms of the production of materials essential 

 for the continued reproduction of those parts 

 (the ends and possibly the substance in the 

 regions of fracture) of the chromosomes that 

 are eliminated in the cells that undergo dim- 

 inution. The site of production would be 

 that region of the cytoplasm halfway be- 

 tween the equator and the vegetal pole of the 

 uncleaved egg, and the formation of cell 

 boundaries would presumably prevent dif- 

 fusion of the material to other cells. 



Chromosome Elimination in Sciara. Another 

 remarkable example of differential behavior 

 of the chromosomes in germ cells and somatic 

 cells occurs in the fungus fly Sciara, studied 

 extensively by C. W. Metz and his co-workers 

 (see Metz, '38, for review, and Berry, '41, for 

 some later details). The zygote starts devel- 

 opment with three pairs of autosomes, three 

 X-chromosomes and, in twelve of fourteen 

 species examined, one, two or three large 

 chromosomes called "limited" chromosomes 

 (Fig. 48). At the sixth division (sometimes 

 the fifth) of the zygote the "limited" chro- 

 mosomes are eliminated from all of the so- 

 matic nuclei. At this stage the nuclei have 

 migrated from the middle of the egg, where 

 the zygote nucleus is originally located, 

 nearly to the periphery. One or two of the 

 nuclei at the posterior pole of the egg form 

 the primordial germ cells, which retain for 

 some time the full complement of chromo- 

 somes present in the zygote. 



Another elimination of chromosomes oc- 

 curs in the somatic cells at the seventh or 

 eighth division. At this time the somatic 

 nuclei of the female-producing eggs elimi- 

 nate one of the three X-chromosomes and 

 the male-producing eggs eliminate two 

 X-chromosomes. In these elimination divi- 

 sions the "limited" chromosomes and 

 X-chromosomes that are to be discarded fail 

 to divide, or divide incompletely. They are, 

 then, left on the middle of the spindle in 

 anaphase and do not become incorporated 

 in the daughter nuclei, but slowly break 

 up and disappear in the later embryo. In 

 Sciara an elimination also occurs later in 

 the germ cells after they have migrated to 

 the site of formation of the gonads, one of 

 the X-chromosomes (one of the two pa- 

 ternal X's) being extruded from each nu- 



