No. I.] EMBRYOLOGY OF THE ISOPOD CRUSTACEA. 69 



The number of ova contained in the brood-pouch varies con- 

 siderably in different individuals ; the smallest number found 

 was three and the greatest twenty-two, the average being 

 about eleven. A moult apparently precedes the oviposition, 

 as has been observed for a number of other species, but I have 

 not endeavored to follow in detail the phenomena accompany- 

 ing the extrusion of the ova. An attempt to discover the con- 

 ditions which governed this process was without result; appar- 

 ently it is not simply a question of time of day, i.e., of light 

 and temperature, nor can the stimulus be supplied by impreg- 

 nation, since in a number of females isolated at the same time, 

 from which later developing ova were obtained, and which must 

 therefore have been impregnated prior to the isolation, the 

 time of extrusion of the ova varied too much to allow of its 

 reference to this cause. 



Attention may be called to the fact that the period of egg- 

 laying of Xh^ Jaera marina of our coast differs materially from 

 that of the European form. According to Sye ('87) the extru- 

 sion of the eggs occurs in the latter in March and April, while 

 at Woods HoU I found that it took place throughout the entire 

 summer. At least it was going on at the middle of June when 

 my first observations were made, and continued without inter- 

 ruption until the end of the first week of September, the time 

 of my departure from the laboratory. 



Eggs taken from the brood-pouch before the polar globules 

 have formed possess but a single membrane, the chorion, 

 which shows a decidedly wrinkled appearance, and is separated 

 from the ovum by a considerable space. The chorion is some- 

 what sticky at this stage, the eggs adhering to each other and 

 to the bottom of the watch-glasses in which they are being 

 examined, but later this property is entirely lost. As to the 

 origin of the membrane, my results are not very definite, since 

 I have not been able to satisfy myself as to whether a delicate 

 membrane was not present in ovarian eggs. In one set of 

 sections through ovaries containing almost ripe ova I thought 

 a very delicate membrane could in some places be dis- 

 cerned, and though the uncertainty was too great to warrant 

 a definite statement, I am inclined to believe that the 



