THE CLEAVAGE OF THE EGG OF VIRBIUS 

 ZOSTERICOLA, SMITH. 



A CONTRIBUTION TO CRUSTACEAN CYTOGENY. 

 FREDERIC P. GORHAM. 



The following investigations were carried on at the Marine 

 Biological Laboratory at Woods Holl, Mass., and at the Biolog- 

 ical Laboratory of Brown University, during the years 1893 

 and 1894. 



Unlike the eggs of most Decapods, those of the "green 

 shrimp," Virbiiis zostericola, Smith, are remarkably favorable 

 for the study of early cell-lineage. The thin chorion, the 

 transparency of the yolk substance, the depth of the cleavage- 

 planes and the early invagination, permit the history of the 

 blastomeres to be followed with great accuracy. The dividing 

 nuclei and the direction of the spindles can be seen in the 

 living ^g^y and if the study of fresh material is supplemented 

 by that of preserved and stained specimens, the result is all 

 that can be desired. 



Females bearing eggs in all stages of development (those of 

 a single individual being of the same stage) can be obtained 

 throughout the summer months and eggs removed from the 

 pleopoda of the female will continue their development in 

 watch-glasses, under the microscope, if the water is occasion- 

 ally renewed. The segmentation and invagination can be 

 watched from beginning to end in a single ^gg, about eight 

 hours being required for the stages which intervene between 

 the appearance of the first cleavage-plane and the completion 

 of the process of invagination ; nuclear division takes place 

 about once an hour. 



No trace of polar bodies has been seen. The early segmen- 

 tation of the egg is total and regular, and at first it is impossi- 

 ble to distinguish between the animal and vegetal poles. The 

 division of the nuclei precedes the division of the yolk and 

 successive spindles are always arranged at right angles to each 

 other. 



