SUTTON 



somes representing any given character 

 would have to be exactly alike, since 

 the combination of any two of them 

 would produce a uniform result. As a 

 matter of fact, however, specific char- 

 acters are not found to be constant 

 quantities but vary within certain 

 limits; and many of the variations are 

 known to be inheritable. Hence it 

 seems highly probable that homolo- 

 gous chromatin-entities are not usually 

 of strictly uniform constitution, but 

 present minor variations correspond- 

 ing to the various expressions of the 

 character they represent. In other 

 words, it is probable that specific dif- 

 ferences and individual variations are 

 alike traceable to a common source, 

 which is a difference in the constitu- 

 tion of homologous chromatin-entities. 

 Slight differences in homologues 

 would mean corresponding, slight 

 variations in the character concerned— 

 a correspondence which is actually 

 seen in cases of inbreeding, where 

 variation is well known to be mini- 

 mized and where obviously in the case 

 of many of the chromosome pairs both 

 members must be derived from the 

 same chromosome of a recent common 

 ancestor and hence be practically iden- 

 tical. 



In the various forms of partheno- 

 genesis we meet the closest kind of 

 inbreeding and a brief consideration 

 of the variability to be expected in 

 each, from the standpoint of the chro- 

 mosome theory, may serve as a guide 

 to such research as will test the valid- 

 ity of the latter. The simplest form, of 

 which chemical parthenogenesis in sea- 

 urchins is an example, is that in which 

 the organism has only a single chro- 

 mosome series, to be represented by 

 A, B, C, D . . . N. Thus far no rec- 

 ognized cases of this type have been 

 reared to sexual maturity, but it is to 

 be expected that no reducing division 

 will be found in the maturation of such 



33 



forms, and that their parthenogenetic 

 offspring will exactly resemble the im- 

 mediate parent. 



In cases of natural parthenogenesis 

 which are accompanied by the reen- 

 trance of the second polar body and its 

 fusion with the egg-nucleus (or its 

 failure to form) there must be a double 

 chromosome series; but we may dis- 

 tinguish two classes according as the 

 reducing process is accomplished in 

 the first or the second maturation divi- 

 sion. ^^ If reduction is accomplished in 

 the first division, one half the chromo- 

 somes of the oogonia are thrown out 

 and lost in the first polar body. The 

 second division, being equational, 

 would result in a polar body which 

 would be the exact duplicate of the 

 egg-nucleus as far as chromosomes are 

 concerned and which accordingly, by 

 its reentrance would add nothing new 

 to the egg-series. The series after 

 fusion would, therefore, be repre- 

 sented by the letters A, B, C, D . . . 

 N + A,B,C^D . . . N.U such a type 

 of parthenogenesis were to follow 

 sexual reproduction, the first genera- 

 tion of offspring might be expected to 

 differ materially from the parent by 

 reason of the casting out, in the first 

 polar body, of chromosomes repre- 

 senting certain dominant characters, 

 and the consequent appearance in the 

 offspring of the corresponding reces- 

 sives. Subsequent parthenogenetic gen- 

 erations, however, would in each case 

 be endowed with a chromosome series 

 exactly similar to that of the immediate 

 parent and accordingly might be ex- 

 pected to show the same characters. 



In case the second division of a par- 



13 Either must be regarded as possible in 

 cases where we have no definite knowledge 

 since it is regularly described as the second 

 in the Orthoptera (McClung, Sutton) and 

 Copepoda (Riickert, Hacker) while in the 

 Hemiptera-Heteroptera it is believed to be 

 the first (Paulmier, Montgomer}') • 



