76 WILLIAM MORTON WHEELER, 



the somatic cells. The germ-cells appear different because they are 

 inert, dividing very slowly or not at all. This accounts for their 

 larger size, and the retention of yolk granules in their cytoplasm. 

 This inertia is clearly shown in Cymatogaster^ of which Eigenmann 

 says (1897, p. 166) : "The number of (sex-)cells originally segregated 

 (9—23) remains unchanged from the time of their segmentation till 

 the larva has reached a length of 7 mm except that four of them 

 are lost, two in the gill-region and two in the middle of the body." 

 The most natural explanation of this singular inertia on the part of 

 the sex-cells is to be found in the fact that the ova and spermatozoa 

 are usually the last elements to be required in the ontogeny of the 

 animal, and the energy of development is first entirely confined to 

 the division and differentiation of the somatic cells. It may well be 

 the case that the number of cell generations from the time of seg- 

 mentation to the formation of a ganglion cell, gland cell or other 

 somatic element, is not greater than the number leading to the 

 formation of the spermatozoon , but the divisions are for obvious 

 reasons hastened in the one case and delayed in the other. It is this 

 difference in the rate of division which brings about the early "dif- 

 ferentiation" of sex-cells so common in certain Invertebrates and lower 

 Vertebrates, 



Does the fact of the early segregation of the sex-cells prove 

 that the ancestors of the Vertebrates did not have metameric sex- 

 organs, or gonads'? I think not, for first in Àmphioxus (1892b) the 

 sex-cells, so far as known, are not apparent till they appear in certain 

 definite portions of the coelomic walls as metameric structures. To 

 these circumscribed and homodynamous regions of the coelomic walls 

 the name gonotome is perfectly apposite. That it is a primitive 

 arrangement is shown by the same condition of the sex-cells in Chœto- 

 pods and certain Insects, the former group having, in my opinion, 

 transmitted this arrangement both to the Chordata and the Arthro- 

 poda. The two conditions of the sex-cells in Vertebrates — the 

 metameric in Acrania and the ametameric in Craniota — are paralleled 

 in the Insecta. Here, too, certain primitive forms like the Ortho- 

 ptera, show the sex-cells, when they first appear, lying in the walls 

 of the mesoblastic somites, and hence metameric, but in several of 

 the more specialized Insects like the Diptera, the segregation of the 

 sex-cells has been pushed back into the very early cleavage stages. The 

 principle of retardation, or inertia, above alluded to, stepping in at 

 very different times in the development of different Insects and Verte- 



