18 THE DEVELOPMENT OF 



saler Mittclclarmblindsack") is comparable to my middle 

 liver lobe. 



It would appear that Keichenbach feit compelled to rec- 

 ognize entoderm in the caual proper, for a digestive tract 

 withoiit entoderm seems an anomaly. Fore- mid- and bind 

 guts are universally quoted, bat their limits are far from 

 recognizable !n either young or adult. Thus in Palsemon 

 according to Bobretzky ('75) as abstracted by Hoyer, p. 

 318, "Die Kern-baltige peripherischeScbichtder denDarm- 

 drüsenkeim bildenden Pyramiden hebt sich als gesonderte 

 Zellenschicht von dem centralen Doltermasse ab, die durch 

 Einschnürung in zwei vordere und zwei hintere Ballen 

 zerfällt, und bis zum Ausschlüpfen der Larve völlig ver- 

 braucht wird. Der eigentliche Mitteldarm schnürt sich 

 durch einfache Faltenbildung von den oberen Wand des 

 Darmkeimes ab, wahrend der Rest des letzeren allmälich 

 in Leberkanalchen sich zerklüftet." This, as will be seen, 

 agrees with my account of what occurs in Crangon, due 

 allowance being made for the different character of invag- 

 ination, except in the cutting off' of the midgut proper. 

 But we are left in doubt as to the extent of this ' eigentliche 

 Mitteldarm.' In this tact that the entodermal cells give rise 

 to the liver is possibly to be sought the source of Nusbaum's 

 view ('87) already adverted to, that the vitellophags of 

 Mysis degenerate. He was looking for the entodermal 

 portion of the alimentary tract in the straight portion of 

 the canal, and being unable to trace them into any portion 

 of this and, failiug to recognize that they gave rise to the 

 liver, he was at a loss to explain their fate in any other 

 way. 



In connection with the method of formation of the ali- 

 mentary tract proper which I have described in the fore- 

 going pages, it is interesting to note the condition which 



