CLEAVAGE AND FORMATION OF THE GERM-LAYERS. 165 



attached to the wall of the uterus, is nevertheless nourished by 

 fluid yielded by that organ. In the New Zealand species, such 

 nourishment from the mother is not needed, since the embryo is 

 not essentially larger than the egg. In this case, therefore, the 

 material for the development of the embryo must be contained in 

 the egg itself. It is actually found that the egg of P. novae- zeal andiae 

 is very rich in yolk, as are those of most Arthropods. The general 

 course of its cleavage also agrees with what is found, for instance, in 

 the Insecta. Considering the close relationship of Peripatus to the 

 Arthropoda, which can hardly be disputed, it seems likely that the 

 condition of the egg of P. novae-zealandiae is the primitive condition. 



It is probable that Peripatus, like the terrestrial Arthropoda generally, origin- 

 ally produced eggs rich in yolk which it laid. This state of things is recalled by 

 the presence of a firmer egg-envelope in P. novae-zealandiae, already pointed out 

 by Sedgwick (No. 11) ; the laying of eggs not fully developed also in this same 

 species points in this same direction, even though we find that eggs laid thus 

 early do not attain full development (Hutton, No. 3). The capacity for 

 developing the eggs within the body must have been secondarily acquired. The 

 ■egg of the New Zealand species, which is rich in yolk and develops within 

 the uterus, represents the first step in this newly-acquired course of development. 

 An accumulation of nutritive material in an egg which develops within the 

 uterus is unnecessary, and this is opposed to the assumption that in P. novae- 

 zealandiae we have a specialised form in which the egg has been secondarily 

 supplied with yolk. A further step in adaptation would be represented by 

 P. capensis. The eggs here show a spongy structure as if penetrated by fluid 

 yolk, and this, as well as the method of their development, seems to indicate 

 that they to a certain extent represent a degenerate condition of eggs originally 

 rich in yolk. Isolated granules of yolk also appear in these eggs, and in P. 

 Balfouri the egg is still somewhat rich in definite yolk-masses. In the species 

 found in the West Indies, the nourishment of the embryo by the mother has 

 become so complete that no trace of the former rich supply of yolk remains in 

 the eggs which have become extraordinarily small. These biological conditions 

 naturally find expression also in the method of development of the different 

 species. * 



1. Cleavage and Formation of the Germ-Layers. 



Although the early development, the cleavage and the formation of the germ- 

 layers, has repeatedly been investigated in different species, our knowledge of 

 these processes still remains very incomplete. This fact is accounted for by the 



* [Willey (App. to Lit. on Onychophora, No. II.), from the study of the 

 «gg of P. novae-britanniae, has come to conclusions which are exactly the reverse 

 of those given above ; and here he is in agreement with v. Kenxel, who 

 believes that the ancestral Peripatus discharged a small yolkless egg into the 

 water, and that the intra-uterine development was concomitant with the 

 adaptation of the parent to a terrestrial existence. These authors then conclude 

 that the development of yolk in the eggs of P. novae-zealandiae is quite a 

 secondary condition, which, Willey believes, culminates in the return to the 

 oviparous condition observed by Dexdy (App. to Lit. on Onychophora, No. I.) 

 in P. oviparus, which Willey regards as a secondarily acquired habit. — Ed.] 



