LITERATURE. 181 



the germ-layers separate. A cell-mass arising from these by division shifts 

 within the embryo, and there forms an unpaired plate lying dorsally above 

 the rudiment of the mid-gut ; this plate only secondarily divides into two 

 halves. The above mass of cells is invested later with a mesodermal envelope. 



The statements of Glaus (No. 9) in connection with the development of the 

 genital organs in Branchipus are of importance. The rudiment of the genital 

 glands is here a paired strand lying in the three or four anterior segments at 

 the sides of the alimentary canal, and recognisable even in early stages. The 

 first development of the efferent ducts, however, takes place at a later stage 

 of sexual differentiation (p. 200), and proceeds from a transformation of the 

 twelfth and thirteenth post-cephalic segment into two pairs of genital swellings. 

 The latter, in the female, join in the median line, while in the male they remain 

 separate. In both sexes the genital swellings of the posterior segment are grown 

 over by those of the anterior segments. The fused swellings then appear, in 

 the female, as a broad median prominence, or in the male as paired lateral 

 prominences. The cell-material found in the genital swellings is utilised in 

 such a way that the mesoderm-cells yield the efferent apparatus (oviduct and 

 uterus — seminal duct and seminal vesicle) as well as the accessory glands, while 

 an ectodermal ingrowth on the second pair of swellings becomes, in the female, 

 the short external portion of the uterus (vagina), and, in the male, the long 

 protrusible copulatory organ (cirrhus). 



In the Decapoda, the genital rudiment has only been observed in the latest 

 stages of embryonic life. Bobretzky and Reichenbach (No. 65) conjecture 

 that it is represented by two cell-strands which lie above the intestinal canal. 

 According to Bobretzky, these lie in the mid-gut region below the pericardial 

 septum, whereas Reichenbach observed the rudiment in the posterior segments 

 in the hind -gut region. 



Nijsbaum (No. 39) found the genital rudiments, in Mysis, as a paired group 

 of cells lying behind the hepatic rudiment. When the hepatic tubes develop, 

 this rudiment shifts towards the dorsal side, and later probably fuses to form 

 an unpaired rudiment lying between the heart and the intestine. It appears 

 doubtful whether a few large cells observed in the ectoderm at the time when 

 the germinal layers form, and which later lie in the abdomen, are really, as 

 authors maintain, to be identified as the genital rudiment. 



Although, for general reasons, we feel inclined to attribute the genital rudi- 

 ments to the mesoderm, Pereyaslawzewa and Rossijskawa (Nos. 70 and 73) 

 derive the genital cells in the Amphipoda from the wall of the mid-gut (!). 

 According to these authors, a few entoderm-cells leave the mid-gut epithelium 

 (in Orchestia, the hepatic tubes also), and, surrounded by a mesodermal envelope, 

 become the rudiment of the genital glands. It is well known that such an 

 origin was asserted for these glands in Peripatus by Sedgwick. 



LITERATURE REFERRING TO THE EMBRYONIC 

 DEVELOPMENT OF THE CRUSTACEA. 



Crustacea in General. 

 1. Beneden, E. van. Kecherches sur la composition et signification 

 de l'ceuf. Mem. cour. et Mem. Sav. Strang. Acad. Roy. 

 Belgique. Tom. xxxiv. 1870. 



