DEVELOPMENT OF PEEIPATUS NOViE-ZEALANDIiB. 249 



exact parallel in the animal kingdom. It is particularly un- 

 fortunate that so many stages are wanting in my material, so 

 that the exact sequence of events cannot be stated with any 

 certainty. It seems strange that the early stages of all the 

 three species of Peripatus should differ so remarkably from one 

 another, while the later course of development seems to be 

 nearly similar in all three. 



It might be said of the mode of development of P. novse- 

 zealandise that the embryo is formed by a process of crystal- 

 lising out in situ from a mass of yolk among which is a proto- 

 plasmic reticulum containing nuclei. 



The two most remarkable features in the development are 

 perhaps the mode of nutrition of the embryo and the mode of 

 formation of the posterior part of the embryo, and it will be 

 more convenient to discuss these two separately. 



Mode of Nutrition of the Embryo. — As has been 

 shown the embryo derives nutriment from two sources, (a) 

 the yolk contained within its body; (b) a peripheral layer 

 of yolk in which are embedded numerous small round, 

 highly refractive bodies. The former of these sources need 

 not be considered, as it is similar to that which occurs in many 

 other eggs, which are loaded with food-yolk, being present in 

 the hypoblast, but entirely absent in the mesoblast. It is with 

 the peripheral yolk that we are concerned. The complete 

 envelopment of the ovum in a thick peripheral layer of yolk is 

 a very remarkable and unusual mode of embryonic nutrition, 

 but its object evidently is to supply the ectoderm with a 

 constant source of nourishment, the yolk first and the small 

 round bodies eventually being completely absorbed by the 

 ectoderm cells. It seems possible to regard it as ectodermal 

 yolk, and it is very probably homologous with the peculiar 

 arrangement in the ectoderm cells of Peripatus capensis 

 which Mr. Sedgwick has described in the region of the 

 hump. He says (These ' Studies,' pt. 1, vol. iv, p. 54,') : 

 "This increase in thickness '^ (i. e. of the ectoderm) "is 

 mainly due to the appearance, outside the nuclei, of a layer of 

 vacuolated protoplasm. The vacuolation is a 



