174 



ONYCHOPHORA. 



ontogeny of Peripatus to be able to make further comparisons. We must, 

 however, add the description given by L. Sheldon (No. 12) of the earlier stages 

 of P. novae-zealandiae, according to which the embryo proper still within the 

 egg-shell is surrounded by a layer of yolk (the ectodermal yolk of Miss 

 Sheldon). Unfortunately no details as to the significance and origin of this 

 " external yolk " are known, but we might in this also see an embryonic 

 envelope, especially as structures resembling nuclei are found in this outer layer. 

 We are led to adopt this assumption all the more on account of the condition 

 of those Insecta (or Myriopoda) in which the germ-band sinks into the yolk, 

 a condition which finally leads to formation of the embryonic envelopes. In 

 consequence of the Insectan embryonic envelopes arising in this way the 

 embryo here also may be apparently surrounded by an outer layer of yolk, 

 which in reality lies between the embryonic envelopes. 



The presence of such envelopes derived from folds is not confirmed by what is 

 found iu P. capensis. In this form nothing of the kind has been observed, nor 

 can we assume that such a feature has been overlooked. The ectoderm of P. 

 capensis is only so far peculiar in that it is in the younger stage extraordinarily 



rich in vacuoles and of a spongy 

 texture, and, in consecpience of 

 its structure, is aide to take 

 in nourishment endosmotically 

 (Sedgwick, No. 10). L. Sheldon 

 connects this structure with the 

 so-called ectodermal yolk of the 

 New Zealand species, but we can- 

 not consider this a happy com- 

 parison. On the other hand, this 

 condition of the ectoderm helps 

 to explain the formation of the 

 external organs of nutrition in the 

 embryos of the American species, 

 whether these are formed direct 

 from the ectoderm of the embryo 

 itself or represent a specially differ- 

 entiated portion of the embryonic 

 envelopes. 



If, in dealing with the early 

 ontogeny of the different species 

 of Peripatus, we have appeared to 

 dwell almost entirely on the rela- 

 tive probabilities of the processes 

 described, we can only again point 

 out how very little is known with 

 certainty of the earliest develop- 

 ment of this animal. The great 

 importance of this form forces us 

 to take into account statements that are not sufficiently confirmed. We have 

 therefore tried to gather together the facts as yet known into a connected 

 whole, but do not for one moment assume that the conclusions arrived at 

 are final. 



Fig. S2.— Sections through [embryos of P. 2m- 

 thnrni at various stages (after Sclater). E, 

 embryo ; a, outer, i, inner cell-layer of the 

 embryo ; m, (cuticular) membrane bounding 

 the uterus internally ; v, placenta-like growth 

 of cells. 



