176 CRUSTACEA. 



sacs forming by constriction at its sides. These hepatic sacs give 

 rise later, by longitudinal constriction, to four or six tubes. In 

 some cases (Oniscus, Caprella, SunampJiithoe), the formation of the 

 hepatic tubes precedes the development of the mid-gut vesicle. In 

 most cases, the chief part of the entoderm is used up in the forma- 

 tion of the hepatic rudiment, only a small part entering into the 

 formation of the central portion of the intestinal canal. The latter 

 is almost exclusively composed of the fore and hind -guts, only 

 a short tract in the immediate neighbourhood of the openings of 

 the hepatic tubes being entodermal in origin. As diverticula of the 

 posterior section of the mid-gut, there arise in Gammarus (PEREYAS- 

 LAWZEWA) those paired tubes (urinary glands], the entodermal nature 

 of which was recognised even in the anatomical researches of 

 NEBESKI. The homologising of these urinary tubes with the Mal- 

 pighian vessels of the Insecta is on this account inadmissible, the 

 latter belonging to the hind-gut, and therefore having an ectoderm al 

 origin. 



The observations of BULLAK (No. 81) on the development of the mid-gut in 

 Cymothoa are in fairly close agreement with those of NUSBAUM for Oniscus. 

 There are, however, no so-called yolk-cells here inside the very considerable 

 mass of food-yolk ; the formation of the mid-gut proceeds from the inner 

 cell-layer of the germ-band. The first trace of a separate entodermal rudiment 

 is found in a paired mass of cells lying somewhat behind the stomodaeal 

 invagination ; this mass of cells gives rise to three pairs of hepatic tubes. 

 At a later stage, a layer of cells connected with the epithelium of the hepatic 

 tubes grows over the whole of the food-yolk. The rudiment of the mid-gut 

 now consists of the yolk-vesicle thus formed and the hepatic tubes communi- 

 cating with it. The anterior end of the yolk-vesicle is connected with the 

 stomodaeal invagination. The proctodaeal invagination, on the contrary, is 

 not in contact with the posterior end of the yolk-sac, but runs forward over 

 its dorsal side so as to pass into it quite near the stomodaeal invagination. 

 The yolk- vesicle appears now as a ventral diverticulum of the alimentary canal, 

 but, as most of it undergoes absorption, it results that here also only quite a 

 small part of the definitive intestinal canal near the points of entrance of the 

 hepatic tubes belongs to the mid -gut (cf. p. 139 on I Agio). 



The above description of the formation of the mid-gut in the Arthrostraca, 

 founded on the observations of BULLAR, NUSBAUM, PERKYASLAWZEWA, and 

 ROSSIJSKAYA, applies, according to NUSBAUM, to Mysis also (Figs. 65 and 66, 

 p. 135). The entoderm here originally lies as a mass of cells in the most 

 posterior section of the germ-band (p. 134). The entoderm-cells soon increase 

 in number, and spread out over the whole ventral surface of the embryo. At a 

 later stage they also reach the lateral and dorsal parts, the food-yolk in this 

 way becoming surrounded by a layer of entoderm-cells While this circum- 

 crescence is going on in the posterior part of the embryo, the entoderm in the 

 anterior part (behind the mandibular segment) forms two lateral grooves 

 consisting of large granular cells (Fig. 66, I) ; these are the rudiments of the 



