No. I.] EMBRYOLOGY OF THE ISOPOD CRUSTACEA. 71 



those of Garmnariis ptilcx. In both these cases, as in Moina^ 

 a membrane forms later which must be identified as the vitel- 

 line membrane. 



In regard to this latter membrane the data as to its occur- 

 rence are not quite so thorough as could be desired, partly- 

 owing to a membrane formed at a much later period, the blas- 

 toderm membrane, having been confounded with it in some 

 cases. It seems necessary to distinguish between the two 

 structures, the vitelline membrane being, in the strict use of 

 the term, a membrane which is formed by the protoplasm of the 

 Q.%^ about the time of the formation of the polar globules, and 

 connected, as will be shown later on, with the process of ferti- 

 lization. It is not my intention to enumerate the various 

 instances in which the occurrence of this membrane has been 

 described, but a few cases may be referred to. In Asellus van 

 Beneden ('69) observed the membrane only when the process 

 of segmentation had advanced to the eight-celled stage, but I 

 have seen it in the egg of A. coinni2inis immediately after the 

 formation of the polar globules. In Onisais Bobretzky ('74) 

 found two membranes in the youngest ova which he obtained, 

 these membranes being apparently the chorion and the vitel- 

 line, but on the other hand Bullar ('78) finds but a single mem- 

 brane in the newly extruded ova of Cymothoa, a second one 

 appearing only when the embryo is fairly formed, there being 

 apparently no true vitelline membrane, though it is possible 

 that it may have been overlooked, since Bullar did not have an 

 opportunity of examining ova in the early stages of segmentation. 



In the Decapod Crustacea there are more definite state- 

 ments as to the absence of this membrane. Ishikawa (85) 

 found two membranes in AtyepJiha, and Lebedinski ('90) has 

 described the same number as occurring in EripJiya, and it may 

 be presumed that in both these cases we have to do with 

 a chorion and a vitelline membrane. In the Lobster, however, 

 Bumpus ('9i) found no trace of a vitelline membrane, and the 

 same result followed the researches of Kingsley ('8?) on 

 Crangon, Cano ('93) on Maja, and apparently those of Mayer 

 ('77) on Eiipagunis and Herrick ('92) on A/pheiis, to mention 

 only some of the more recent observations. 



