-524 PROCEEDINGS OF THE ACADEMY OF [DeC, 



der zwei ersten J^lastomeren, die Selenka l)ci Thysaiiozoon luid Eur}-- 

 lei:»ta coiistatirte, niclit nur bei Discocclis tigrina, sondern auch bei 

 alien Pseudt)ceriden unci Eurylepta nachweisen konnen. Ich glaube, 

 dass sie auch bei alien Leptoplaniden existirt, obschon sie hier schwer 

 nachweisbar ist." 



Since the polar l)odies do not remain attached to the animal pole, I 

 have been unable to ascertain whether there is a rotation of the spheres, 

 indicating that the first cleavage is spiral, as Conklin (97) has shown 

 for Crepidula. With the separation of the two cells their outlines 

 become more or less irregular. Especially along their line of contact 

 <lelicate protoplasmic processes extend outward. This is very much 

 less marked than in the case of the maturation divisions, but there 

 seems little doubt but that it is due to the same internal causes, what- 

 ever those may be. The same phenomenon may also be observed in 

 several of the next succeeding cleavages, i.e., at the four- or even 

 eight-cell stage. But with each successive cleavage the processes are 

 smaller and more delicate and the phenomenon less marked. 



The Second Cleavage — Two to Four Cells. 



A little less than an hour after the first two cells have separated a 

 furrow appears in each. These furrows often appear sinuiltaneously, 

 but in many cases one cell begins to divide in advance of the other. 

 Lang finds that in Discoccelis this is always the larger. " Die Theilung 

 erfolgt aber nicht ganz gleichzeitig, die grossere Furchungskugel theilt 

 sich vielmehr etwas friiher als die kleinere." In Planocera this suc- 

 cession is not so marked as in Discoccelis. In mau}^ cases the two 

 spindles are in the same phase at the same time. 



The cells resulting from this second cleavage are much more unequal 

 in size than those of the first division. Each of the U\o cells buds off 

 a smaller cell in a Iseotropic direction (PI. XXXV, figs. 5 and 6). As 

 Lang has found in Discoccelis, the two spindles of this division cross at 

 an angle. Viewed from the side these spindles form an X (fig. 5). It 

 thus happens that the two smaller cells lie at a higher level than the 

 two larger. The former tend to meet in a point at the animal pole 

 (fig. 6). Before the next division they mo\'e downwards a short 

 distance, so that a portion of the first furrow is visible between them 

 and forms a short polar furrow. Viewed from the vegetative pole the 

 two larger blastomeres always meet in a line the so-called vegetative 

 ])olar furrow ("Brechungslinie," Rauber (82), or " Querf urche," Rabl 

 (79)). As in the case of all dextral mollusks and in annelids, this polar 

 furrow turns to the right when viewed in the plane of the first cleavage. 



