358 W. L. SCLATEE. 



Sedgwick in P. capensis; nor would one suspect, from the 

 nature and size of the ovum, that it had been derived from a 

 meroblastic ovum, and had only comparatively recently lost 

 its yolk. 



The only other instance of a holoblastic and alecithal ovum 

 which has been derived from a meroblastic and telolecithal 

 ovum, of which we have any knowledge, is the ovum of the 

 placental mammals which has doubtless passed through a mero- 

 blastic and telolecithal stage such as is now presented by the ovum 

 of Ornithorhyrichus, our knowledge of which is due to Caldwell. 



The results on the mammalian ovum of the loss of yolk are 

 (1) a large diminution of the size of the ovum ; (2) total seg- 

 mentation ; (3) the formation of a blastodermic vesicle which 

 corresponds to the yolk-sac of meroblastic forms, the embryo 

 proper being formed from only a small portion of the original 

 mass of segmentation spheres which is attached to one side of 

 the blastodermic vesicle. 



Now, it seems to me that the loss of yolk has had precisely 

 the same effect on the ovum of Peripatus that it has on the 

 ovum of placental mammals, i.e. (1) diminution of the size of 

 the ovum ; (2) total segmentation, and (3) the formation of what 

 I have termed the embryonic vesicle, which appears to me to 

 be exactly analogous to the blastodermic vesicle of mammals. 



The ovum of Peripatus has a stage directly comparable to 

 the gastrula and blastopore stage of Van Beneden, by means of 

 which the embryonic mass proper is separated from those cells 

 which form the wall of the embryonic vesicle, and it is because 

 this stage in Peripatus, as in the mammal, has nothing to do 

 with the true gastrula and blastopore, such as is found in 

 Amphioxus, that I have termed it the pseudogastrula stage. 



It seems to me that this point is one which has never been 

 sufficiently dwelt upon in morphology, namely, that like results 

 may be due to the same mechanical cause, 1 and that it is not 



1 Professor Lankester some years ago (' Aim. and Mag. of Nat. Hist.' 1870) 

 discussed this point and applied the terms " homoplasy " and " homoplastic " 

 to such resemblances as distinguished from those due to heredity for which 

 he proposed the terms " homogeny " and " homogenetic." 



