No. I.] EMBRYOLOGY OF THE ISOPOD CRUSTACEA. 1 25 



sac becomes drawn out on its posterior surface into three finger- 

 like processes, while in the Oniscidas and Asellidae only two 

 processes are developed, and appear at a somewhat later stage 

 of development. These processes give rise to the liver-lobes 

 of the adult, continuing to increase in length with the growth 

 of the embryo. For a considerable time the liver-lobes lie free 

 in the yolk, eventually uniting anteriorly with the posterior 

 end of the stomodaeal invagination, by which the anterior por- 

 tion of the digestive tract, as far back as the posterior ex- 

 tremity of the stomach, is formed, while the intestine is formed 

 almost completely by the proctodaeal invagination. 



In other words I find, as Bobretzky ('74) found in the case of 

 Oniscus and as Nusbaum ('93) also finds, that the digestive 

 tract is formed almost altogether from the two ectodermal 

 invaginations, the mesenteron being represented principally by 

 the liver-lobes, only a very small portion of the intestine, just 

 where the liver-lobes unite with it, being possibly endodermal. 



The stomodaeal invagination makes its appearance relatively 

 early, and when first clearly distinguishable lies almost on a 

 level with the rudiments of the antennules, if anything a little 

 behind them, slightly later (Figs. 69 and 70 st) being distinctly 

 behind them, though in front of the antennae. The invagina- 

 tion presses deeper and deeper into the yolk, and at its pos- 

 terior extremity enlarges to form the stomach, the liver-lobes, 

 as already stated, uniting with it at its posterior extremity. 

 The proctodaeum appears slightly later than the stomodaeum 

 and when first observable appears as a patch of cells lying a 

 short distance behind the teloblasts (Figs. 62 and 63, a) and 

 therefore having the same relation to these structures as had 

 the blastopore in a much earlier stage, though from the mode 

 of growth of the teloblasts backwards over the blastoporic 

 cells, there can be no identity of these latter with the procto- 

 daeal cells. The invagination at first forces its way through 

 the yolk, but tends towards the dorsal surface of it where it lies 

 in the more anterior abdominal segments. It extends far 

 enough forward to unite or almost unite with the stomodaeum, 

 a very small amount of endoderm, as already stated, possibly 

 being interposed between the two invaginations. 



