TEE MALAYAN SPECIES OF ONYCHOl'HORA. 55 



but the compouud systems of yolk-masses have totally 

 disappeared^ unless they are represented by the nucleated 

 masses. The nuclei of the peripheral layer ai e almost in- 

 variably circular or oval in shape^ and usually much larger 

 than either mesodei'uial or ectodermal ones. Their lai'ger 

 size, most probably, is correlated iu some way with the 

 function of presiding over the transformation and elaboration 

 of the yolk present in the central m-ass of the previous stage. 

 This increase of size is quite comparable to that which takes 

 place in the case of the nucleus of the ovarian ovum, which 

 supervises the process of elaborating and storing up the 

 deutoplasm in the egg-cell. The two processes seem to be 

 comparable in every respect. The nuclei found in the central 

 mass of amoaboid wandering cells, may occasionally present 

 an irregular form, but they seem never to break up and dis- 

 integrate. The wandering character of these cells more than 

 suffices to account for the angular outline of some of their 

 nuclei. In a slightly later stage in the development the 

 angularity of the endodermal nuclei becomes much more 

 marked, even those of the peripheral layer presenting the 

 same characteristic (PI, 7, figs. 11a, 11 &, 11 c, and lie?). 



The Fourth Embryo. — The fourth embryo, the internal 

 structure of which will be described, is the one illustrated 

 by the corresponding figure on PI. 5, and represented by 

 drawings of ten transverse sections on PI. 7 (figs. 12 a, 12 6, 

 12 c, 12 d, 12 e, 12/, 12 g, 12 h, 12 j, and 12 k). 



On the dorsal and ventral surfaces, the ectoderm presents 

 the same characteristics as in the third embryo, already 

 described, with the difference that they are still more 

 emphasised, the layer being thinner, the nuclei more com- 

 pressed, and the space — especially on the dorsal surface — 

 over which it is iu contact with the endoderm, being greater, 

 the last result being brought about, not so much by the 

 withdrawal of the mesoderm, as by the growth of the 

 embryo, and consequently more highly arched condition of 

 the dorsal aspect (PI. 7, figs. 12 e and 12/). 

 - The nervous system has undergone only a very slight 



