OF ARTS AND SCIENCES. 61 



arrangement gives the cleavage-lines a somewhat raoniliform appear- 

 ance. They are not seen in cleavage-grooves until the division is 

 completed. They are unstained. We have not seen these in the 

 living ovum, bat think they may have been overlooked, as the blasto- 

 disc is usually seen from the inner surface, from which the vacuoles 

 would not be easily recognized. A third class of small spherical vac- 

 uoles may be seen at different depths throughout the blastodisc. 



Balfour found in the elasmobranch ovum vacuoles similar to the 

 second chiss.^^ The presence of these vacuoles, he says, gives the 

 cleavage-furrows a '• beaded " appearance. " Their appearance is that 

 of vacuoles, and with these they are probably to be compared. There 

 can be little question that in the living germinal disc they are filled 

 with fluid. In some cases, they are collected in very large numbers 

 in the region of a furrow. Such a case as this is shown in Plate I. 

 Fig. G h. In numerous other cases they occur, roughly speaking, 

 alternately on each side of a furrow. Some furrows, though not 

 many, are entirely destitute of these structures. The character of 

 their distribution renders it impossible to overlook the fact, that these 

 vacuole-like bodies have important relations with the formation of the 

 segmentation f/irroics." 



The " spaltartiger Riiume" described by Oellacher^'^ differ widely 

 in appearance and position from this class, but may nevertheless be of 

 a similar nature. They appear, however, more like the " differentiated 

 plane " which precedes and marks the course of a cleavage-groove. 

 Flemming ^^ has described and figured vacuoles much more closely 

 resembling those we have described. 



About the time the second cleavage begins, we find minute opaque 

 gramdes along each side of the first plane of cleavage. These are 

 most distinct and most numerous towards the extremities of the plane. 

 At the deeper part of the groove, just where this stops and is replaced 

 by the precleavage line above mentioned, the granules are arranged in 

 two linear and parallel rows, one on each side the dividing line. At a 

 somewhat higher focus, their arrangement is much less regular. These 

 gramdes are neither abnormal nor artificial. As to their nature and 

 origin, our preparations give us no definite information ; but they are 

 probably equivalent to the " cell-plate " of Strasburger and the thick- 

 enings of the "interzonal filaments " described by INIark (1, c, p. 231). 



^1 Balfour. Development of Elasmobranch Fislies, p. 13. 

 5- Oellacher. " Beitriige zur Entwicklungsgeschichte der Knochenfiscbe." 

 Zeitsclir. f. Wiss. Zool, XXII., pp. 394, 395, 1S72. (PI. XXXIII. Tig. 22.) 

 °2 Flemming. Zellsubstauz, Kern und Zelltheilung. 



