1878.] 



the Development of the Parasitic Isopoda. 



285 



dorsal surface being very thin and difficult to recognise, while on the 

 opposite side they form a thickened patch (the future ventral wall of 

 the embryo), which elongates, and soon shows traces of segmentation. 



The procephalic lobes appear first, the other segments are formed in 

 order from before backwards. 



A second membrane is now formed round the egg. 



The epiblast of the procephalic lobes soon becomes thickened, to form 

 the cerebral ganglion. At the most anterior end there is a separate 

 mass on each side, but in front of the mouth the thickening is con- 

 tinuous right across ; it extends backwards, along the median line, to 

 form the ventral nerve-cords and ganglia. 



The mouth and, rather later, the anus appear as involutions of the 

 epiblast. 



Beneath the epiblast a second layer of scattered cells appears. 



The limbs arise as hollow protuberances of the epiblast, filled with 

 cells from the lower layer. 



On the dorsal surface a peculiar organ, homologous with the 

 " micropyle apparatus" of the Amphipods, is developed. At a later 

 stage it is situated in the first thoracic segment. It disappears before 

 the adult form is reached. 



Just behind the mouth involution a solid mass of cells appears, 

 from which the liver will be developed. 



The nervous system, which at first consisted of a continuous mass of 

 epiblast cells, becomes differentiated into a cellular and a fibrous por- 

 tion. The external layer of cells or epidermis is separated from it, and 

 it becomes segmented so as to form a chain of separate ganglia. 



The epiblast covering the head is thickened at a certain point on 

 each side, and from these thickenings the eyes are developed. 



The solid masses of cells representing the liver become converted 

 into three ceecal tubes, on each side of the body, opening to the yolk, 

 and quite distinct from both the fore and hind guts. 



The heart appears above the hind gut in the abdomen. 



The hind gut grows forwards very rapidly, and comes nearly into 

 contact with the fore gut. The liver caeca increase in size and become 

 filled with oil-drops derived from the yolk. 



It is now possible to demonstrate, by dissection, that the yolk is 

 surrounded by an exceedingly thin membrane, continuous with the 

 walls of the liver caeca, and opening into the point of junction of the 

 fore and hind guts. This membrane appears to contain nuclei. 



As development proceeds the yolk gradually disappears, and the 

 membrane surrounding it disappears- also. 



The whole of the alimentary canal of the adult is formed from the 

 fore and hind guts. 



In the paper an attempt is made to prove the truth of the sugges- 

 tion, originally put forward by Dr. Dobrn, that the yolk membrane is 



u 2 



