250 LILIAN SHELDON. 



very striking feature. The surface of the dorsal ectoderm, 

 particularly of the hump, is very rough in these stages, and in 

 the best preserved embryos without a definite external boun- 

 dary. It presents very much the appearance which a bath- 

 sponge would present in section, fraying out, as it were, into 

 the surrounding fluid ; and one may fairly conclude that 

 during life it possesses the power of sending out processes into 

 the fluid surrounding the embryo, and that the superficial 

 vacuoles open to the exterior. In short, I am inclined to think 

 that this surface ectoderm during stages e to f has a nutritive 

 function, absorbing the fluid in which the embryo lies, and it 

 seems to me conceivable that the placenta described by Kennel 

 in the Trinidad species may be a more specialised organ of the 

 same nature." It seems also conceivable that the peripheral 

 layer of yolk in P. novje-zealandise may be a more 

 specialised organ of the same nature, and that originally when 

 the ovum of P. capensis was provided with yolk, the space 

 between the egg membrane and the ectoderm was filled with 

 yolk, as is the case in P. novse-zealandiae, instead of as now 

 with fluid. I have not been able to find processes on the 

 external surface of the ectoderm cells, but the boundary is not 

 very sharp, and the protoplasm passes without any definite 

 limits into the peripheral layer. This is specially the case in 

 the stages in which the internal yolk is divided by a longitu- 

 dinal horizontal septum (vide fig. 17 b and 17 c) . The modes of 

 nutrition of Arthropod embryos are, as is well known, very 

 variable, and an arrangement somewhat comparable to this is 

 described by Ganin^ as existing in Platygaster, where a layer 

 of protoplasm containing nuclei surrounds the embryo, both the 

 protoplasmic layer and the embryo being derived from precisely 

 similar elements. He describes the first nucleus as arising as 

 a new formation in the egg ; from this another nucleus arises 

 by division, and from this second one a third. The original 

 nucleus gives rise by a process of complete segmentation to the 

 embryo, the two later ones undergo division, and becoming sur- 



1 Ganin, M., " Beitrage zur Erkenntniss der Entwicklungsgeschichte bei 

 den Insecten," ' Zelt. fiir wissen. Zool.,' xix, 1869. 



