YOLK-COED. 117 



Like all the other chambers ; it is formed of mem- 

 branous walls studded with epithelial scales. Within 

 the terminal chamber, several nucleated cells are visible, 

 which are embedded in the periplast. These cells, 

 have, according to Brandt, an amoeboid movable 

 nucleus, and correspond to the ordinary germinal 

 vesicles with their usual spot. After a time a constric- 

 tion forms at the lower end of the terminal chamber, 

 and one of these germ-vesicles passes through, and 

 then it greatly increases in size. This body finally 

 becomes the pseudovum, which is composed of a pseudo- 

 vitellus encompassed by its nucleated cortex ; which 

 Prof. Huxley likens to the blastodermic layers of the 

 true eggs. 



The resemblance between the pseudovum and the 

 ovum is completed by the formation of the vitelline 

 membrane which completely envelops the pseudovum. 

 The blastoderm separates into two portions, the outer 

 of which forms a sort of hood over the inner. This 

 hood eventually becomes the abdomen of the larva. 

 Other parts of the blastoderm, in a similar way, separate 

 into the thoracic and cephalic segments. 



" The most probable hypothesis as to the nature of 

 the process effecting these changes is that the endo- 

 plast of the pseudovum (germinal spot) divides and 

 subdivides so as to give rise to the endoblasts of the 

 germ. This is more in accordance with what we know 

 of histological development."* 



Brandt endorses all the principal views expressed by 

 Prof. Huxley as above sketched, and, indeed, thinks 

 that the resemblance between the early products of 

 the viviparous and oviparous females of Aphis is so 

 marked, that the terms pseudovum, &c, might be 

 dropped altogether, t 



A few remarks may here be added as to the signifi- 

 cance of the yolk-cord discovered by Prof. Huxley in 

 Aphis, and by others in Coccus and Psylla. It may 



* Huxley, " Reprod. of Aphis," ' Linn. Trans.,' 1858, 

 f Brandt, ' Ueber das Ei ; ' vide note, p. 55, also p. 56. 



