THE MALAYAN SPECIES OF ONYCHOPHOI.'A. 75 



gardiut( tlie changes which take place during the later stages 

 in the development. 



The endodermal elements are derived from the lips of the 

 blastopore, and travel inwards along the outer layer of the 

 yolk, which at first is devoid of nuclei (PI. 6, fig. 9 c, en.). 

 In the second embryo illustrated in figs. 10 a — j on PI. 6 

 the endoderm forms a complete layer, in which the nuclei, 

 especially towards the dorsal aspect, are placed at a con- 

 siderable distance from one another. The central yolk is 

 still free of nuclei. In the third embryo, sections of which 

 are represented in figs. 11 a — d on PL 7, the endodei-mal 

 elements have invaded the central mass, and changed its 

 entire character into one resembling the peripheral layer of 

 the second embryo. In the fourth embryo, which is more 

 advanced than the third, the endodermal elements, leav- 

 ing behind them a number of yolk masses in the centre, 

 have re-entered the peripheral layer. That such is the 

 case is almost certain, from the irregular disposition of the 

 endodermal nuclei, Jind from the absence of degenerating 

 nuclei either in the previous or the present embryo. In the 

 next embryo, the seventh in the uterus counted from the 

 ovary, the endoderm has reconstructed itself. Blood-spaces 

 have already appeared, and the intestine is surrounded by a 

 layer of mesoderm. The endodermal nuclei are placed rather 

 far from one another, and close to the mesodermal coverinar of 

 the intestine. The cytoplasm of the endodermal layer is sparse, 

 vacuolated, and seems to be undivided; and the whole layer 

 is devoid of any kind of food material. In an older embryo, 

 the ninth in the uterus, the general arrangement is the same 

 as in the seventh. The endodermal nuclei are situated at the 

 base, but are closer to one another ; and the cytoplasm, which 

 is still undivided, contains a number of small round bodies — 

 presumably food material. In the next embryo, the tenth in 

 the uterus, the general arrangement resembles that of the 

 previous stages in the development. The nuclei are still 

 basal, but the cytoplasm is distinctly divided by cell outlines ; 

 and yolk-bodies are muckniore numerous and larger than they 



