281 



shows a number of nuclei observed in a single field of the 2 mm 

 immersion lens. The same field included many other similar cases 

 and the same is true of other regions of the imaginal structures. 



In the developing imaginal structures of culex similar con- 

 ditions exist. 



Cicindela. Examination of slides and imbedded material of 

 larval and early pupal stages kindly presented by Mr. V. E. Shel- 

 FORD showed a similar absence of mitosis. Amitosis was less readily 

 observed than in the fly larvae, apparently because of the extreme 

 delicacy of the nuclear membrane, but I was able to find many con- 

 vincing cases. 



Chordata. 



Amphioxus lanceolatus. In an account of certain cell 

 migrations in larvae of Amphioxus Zaknik ('05) expresses himself 

 as follows: "Wie und woher restituiert und regeneriert sich wieder das 

 Leberepithel? Eine sichere Antwort auf diese Frage konnte ich aus 

 meinen Serien nicht erhalten, da ich kein einziges Mal Mitosen, die 

 uns allein Aufschluß über Wachstumsvorgänge erteilen können, in der 

 Leber oder im Darmepithel finden konnte, was ja niemand, der einiger- 

 maßen mit Amphioxus vertraut ist, wunder nehmen wird, denn man 

 findet bei Amphioxus für gewöhnlich absolut keine Mitosen ; woher das 

 kommen mag, will ich dahingestellt sein lassen." 



This rather remarkable Statement led me to examine sections of 

 Amphioxus larvae. Most of my observations thus far have been con- 

 fined to a stage of about 12 mm in length. The nuclei though small 

 are remarkably distinct and the clearness with which conditions that 

 I am unable to interpret as anything but amitosis can be observed 

 renders it surprising that they have not been observed before. 



Nos. I — VIII in Figure 8 are from the general ectoderm. Cases 

 of this sort were most frequent in the atrial folds which are in pro- 

 cess of formation at this stage; Nos. IX — XII are from the central 

 nervous system; Nos. XIII — XVII from the branchial apparatus, and 

 Nos. XVIII — XXI from the intestine. The number of figures might 

 be multiplied almost indefinitely even from a single section. I was 

 unable to find a single case of mitosis. Such cases as this leave no 

 doubt that amitosis is a characteristic feature of normal development 

 in Amphioxus. It is only total neglect by cytologists of such cases as 

 this which has made possible the belief in the all-importance of mitosis. 



Squalus acanthias. In embryos of Squalus 13 mm in length 

 mitoses are not at all uncommon. But in addition to these and in 



