18 THE DEVELOPMENT OF 



saler Mitteldarmblindsack") is comparable to my middle 

 liver lobe. 



It would appear that Reichenbach felt compelled to rec- 

 ognize entoderm in the canal proper, for a digestive tract 

 without entoderm seems an anomaly. Fore- mid- and hind 

 guts are universally quoted, but their limits are far from 

 recognizable in either young or adult. Thus in Palsemou 

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

 318, "Die Kern-haltige peripherische SchichtderdenDarm- 

 driisenkeim bildenden Pyramiden hebt sich als gesonderte 

 Zellenschichtvon dem centralen Doltermasse ab, die durch 

 Einschnurung in zwei vordere und zwei hintere Ballen 

 zerfallt, und bis zum Ausschliipfen der Larve vollig ver- 

 braucht wird. Der eigentliche Mitteldarm schntirt sich 

 durch einfache Faltenbildung von den oberen Wand des 

 Darmkeimes ab, wahrend der Rest des letzereu allmalich 

 in Leberkanalchen sich zerkliiftet." 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 Ave are left in doubt as to the extent of this ' eigeutliche 

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

 to the liver is possibly to be sought the source of Nusbauin'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, failing 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 



