524 ElOHARD EVANS. 



the adoption of such a course. In fact, it seems a decided 

 disadvantage for an egg wliicli develops inside the mother, 

 within easy leach of any amount of food material in the form 

 of secretion from the tubular walls of the oviducts and uteri, 

 to have to move along from the ovary to the uteri carrying 

 with it a mass of yolk of an immense size, of which it has no 

 need. The improbability of the view held by Kennel and 

 Willey when applied to Eoperipatus, the young of which 

 measure 22 to 27 mm. in length at birth, is so great that 

 we are forced to reject it, and to adopt Sedgwick's view 

 of the primitive condition of the ovum in the ancestral 

 form. 



Having reached this position by a method of reasoning 

 which appears to be perfectly legitimate, the explanation 

 which Sedgwick gave of the structure of the ovum in P. 

 capensis follows naturally, and the ovum of Peripatoides 

 would have to be explained in the same way as that of 

 Eoperipatus. The next conclusion of necessity follows, 

 which is, that the oviparity of P. oviparus is primitive and 

 not secondarily acquired. 



To sum up, the following seem to be the stages in the 

 evolutionary changes of structure of the ovum, as its 

 development became more and more confined to the uterus. 

 Peripatoides represents a primitive condition, and produces 

 a yolk-bearing egg, which either develops within the uterus to 

 a small embryo, or is discharged and develops outside. The 

 second step is met with in Eoperipatus, which has a very 

 large yolk-bearing egg, from which is developed an embryo 

 measuringfrom 22 to27mm.in length at birth. Peripatopsis 

 supplies the third step, with a large egg possessing highly 

 vacuolated cytoplasm, and produces an embryo of medium size 

 in the uterus. Paraperipatus represents the fourth step, 

 with a yolkless egg, much reduced in size, and gives rise to 

 an embryo of medium length. The genus Peripatus seems 

 to present the culminating point in these changes, for it not 

 only produces a yolkless egg, but seems to possess what is a 

 highly modified mode of development. 



