THE ANATOMY AND DEVELOPMENT OF PERIPATUS XOVAE-BRITANNIAE. 33 



secondary or accessory function. The amniotic folds of Insects are purely protective. 

 Thus the primitively secondary function of protection has entirely superseded the 

 original nutritive function. It follows from this view that the amnion of Insects is 

 not a new acquisition of their yolk-laden eggs, but is the derivative of an original 

 nutritive organ developed in correlation with an alecithal ovum. 



The trophic organ of the embryo of P. novae-britanniae is analogous to a temporary 

 larval structure — like the suckers of a tadpole. But unlike the latter it is not developed 

 for use during an independent larval existence, but is essentially developed for use 

 during uitra-uterine development. When the embryo is flexed and the anterior portion 

 of the trophic organ covers its ventral surface as with a cap (see Text-figure 7) the 

 superficial resemblance to the amnion of an insect is remarkable — and this resemblance 

 must, in principle, be still more remarkable in the case of those insects (certain Diptera, 

 see Korschelt and Heider, p. 783) in which the amniotic folds do not fuse together, but 

 remain as separate folds 1 . This is the case, according to Kowalevsky and Graber (quoted 

 by Korschelt and Heider) in the Muscidae "bei denen die Kopffalte ausserst rudimentar 

 bleibt und nur die Schwanzfalte zu etwas deutlicherer Entwicklung gelangt. Bei der 

 spateren Ausbildung des Embryos werden diese Falten einfach wieder ausgeglattet und 

 nehmen dann, wie es scheint, an der Ausbildung der Riickenhaut einen gewissen Antheil." 

 This is exactly what the trophic folds of the embryos of P. novae-britanniae do. In the 

 latter, however, the anterior trophic extension or head-fold predominates considerably, 

 through all stages, over the posterior or tail-fold. There is thus a temptation to suggest 

 that, in addition to the superficial resemblance, there is a genetic relation between the 

 trophic organ or trophic folds here described and the amniotic folds of Insects. And this 

 I do tentatively suggest, on the -nine principle which led Hubrecht to reject the 

 prevailing grossly mechanical explanation of the amnion of the higher Vertebrates, and 

 to trace it back to a primitive trophic organ, the trophoblast (Hubrecht 9). 



Lecithai.itv of the Ovum. 



It is perhaps not always realised that the acquisition of yolk is as radical an 

 innovation as that of any other kind of trophic organ for the nutrition of the embryo. 

 The origin of yolk has been the subject of as much controversy as any other problem 

 of embryology. Even now there seems to be no prospect of arriving at an agreement, 

 not so much as to the origin of yolk in any concrete example, but rather as to the 

 general principles which govern the acquisition and loss of yolk. The acquisition of 

 yolk is an observed phenomenon within the limits of many groups of animals. The 

 loss of yolk in any specified case is always an assumption or hypothesis. 



I will not attempt to discuss this very difficult subject about which hardly any 

 two zoologists hold similar opinions, but will merely point out how the question is 

 affected by the phenomena of development observed in P. novae-britanniae. Id 



1 The figures of the embryos of Lepisma given by Heymons should be compared with those of the embryos 

 of P. novae-britanniae accompanying this paper, so far as external appearance is concerned. According to 

 Heymons, the amniotic cavity in Lepisma never completely closes during the period of its existence but has 

 a permanent external opening, the amniotic pore (Amnionporus). [Richard Heymons. Entwicklungsgesehichtliche 

 Untersuchungen an Lepisma saccliarina L. Z. f. w. Z. Bd. 62, 1897, p. 583.] 



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