MUSEUM OF COMPARATIVE ZOOLOGY. 243 



that this must be the cause of its further metamorphoses. Finally, 

 during the division of the yolk, the clear spot becomes fully divided 

 into two parts, each with a trail, like the tail of a comet, stretching 

 away toward the last point of contact. The trails soon disappear, and 

 the spots gradually assume the globular form, though not yet capable 

 of being isolated from the yolk, from which he concludes the nucleus has 

 not yet a distinct membrane. 



Accompanying the subsequent approach and mutual flattening of the 

 cleavage spheres * the nuclei acquire a membrane. That Warneck had 

 observed very closely the formation of the new nuclei must be evident, 

 I think, from the following passage (p. 138) : " Im Anfange .... sind 

 sie [die Kerne] noch sehr klein (noch kleiner sogar, als diejenige helle 

 Masse, welche in jede Kugel sich absondert) und fast glaube ich, dass 

 nicht die ganze Masse unmittelbar in jede Kugel [jeden Kerni] iiber- 

 geht, sondern nur ein gewisser Theil derselben wird Anfangs, gieich 

 einem Centrum, von einer Hiille umgeben und vergrossert sich dann auf 

 Rechnung der sie umgebenden Masse." 



Gegenbaur ('51, pp. 373, 374) has only incidentally touched upon 

 some of the questions which interest us, his main purpose being the 

 investigation of the genesis of the organs and their tissues. The ab- 

 sence of a cell membrane, owing to the essential importance at that 

 tjme attached to this structure, seems to have caused him to entertain 

 a less just conception of the cell nature of the cleavage spheres than 

 was held by Warneck ; for he remarks that only a negative result was 

 obtainable in regard to the question of the cell nature of these spheres, 

 inasmuch as he was in no way able to demonstrate a membrane, and 

 therefore concludes that these spheres are to be considered as in the 

 process of being generated into cells, — "als angelegte Zellen," — which 

 only attain their formation into genuine cells after oft-repeated division. 



The so-called " Richtungsblaschen " arises through the elimination 

 {Ausscheidung) of a little drop of yolk substance, which often contains 

 various globules of fat, and often remains a long time in the vicinity of 

 the yolk without acquiring any further relation to the embryo except that 



* Of the phenomena which are observable during or immediately subsequent to 

 this flattening, already very well described by Warneck, I will limit myself here to 

 the statement that in Limax campestris the elimination of the secreted fluid takes 

 place at the animal pole. 



Iwould also say concerning a subsequent phase of segmentation, that in L. cam- 

 pestris the stage consisting of 8 cleavage spheres is not followed by one of 12, but 

 both large and small spheres divide at the same time, so that the stage of 16 spheres 

 follows directly that of 8, 



