168 



ONYCHOPHOIU. 



Arthropods, with an egg very rich in yolk. "Whether the blastoderm is really 

 formed by eircumcrescence of the egg starting from one pole, or whether the 

 nuclei contained in the yolk, by shifting to the surface, help to form it, the 

 peripheral accumulation of cells which recurs in the same way in various stages 

 claims identification with the cell-accumulation in the neighbourhood of the 

 blastopore {cf. Figs. 76 and 77). We should then not be obliged to assume 

 gastrulation through epibole, which is unusual in eggs so rich in yolk, but 

 should rather assume that at the point where this cell-accumulation is found 

 a depression (invagination) occurs (Fig. 77 B). Whether the base of this 

 depression is formed of yolk (containing nuclei), or whether a closed archenteron 

 is present, would in this case still have to be decided. If the blastopore 



lengthened later {cf. also P. 

 capensis) there would be a 

 resemblance to the gastru- 

 lation of the Insects. 

 In these latter, as in Peri- 

 patus. the mouth and anus 

 show a connection with the 

 two terminal points of the 

 long blastopore. 



In this conception of the 

 cleavage and formation of 

 the germ-layers, it may be 

 noticed that the invagina- 

 tion apparently takes place 

 at the animal pole of the 

 egg. But if it is remem- 

 bered that in P. capensis 

 the brain arises in the im- 



B mediate neighbourhood of 



• ^-^^ W. the blastopore, it will be 



seen that we must rather 

 regard this as a shifting, of 

 the vegetative pole, or the 

 region of entoderm-forma- 

 tion, towards the animal 

 pole, than as a gastrulation 

 at the animal pole. The 

 same is the case in the 

 Insecta and in many Crus- 

 icea (Vol. ii., pp. 141 and 

 142). The view here adopted 

 receives a general support 

 from the conditions in the 

 Crustacea, in which the eir- 

 cumcrescence of the yolk (or 

 the formation of the blasto- 

 derm) takes place from one 

 point, gastrulation after- 

 wards occurring in that 

 region (Vol. ii., p. 115). We are unable in the present state of knowledge to 

 obtain any light upon these processes from the ontogeny of P. capensis. 



Fig. 77.— Sections through the egg of P. novae-zealand 

 showing the formation of the blastoderm and invagina- 

 tion (after L. Sheldon), hi, blastopore. 



