44 SEGMENTATION AND FORMATION OF THE LAYERS. 



favorable for study, because of their large size and the 

 rapid changes which they undergo. Unfortunately I have 

 not, so far, been able to make out the sequence of these 

 changes. 



It is a disputed point as to whether or no the nuclear and 

 extra-nuclear reticula are continuous. Leydig(12), Strieker 

 (16), Klein (9, 10, 11), and Heitzmann (5), hold that they are. 

 So far as the nucleus of the early segmentation stage, and of 

 the endoderm of Peripatus is concerned^ I am able fully to 

 confirm the views of these observers. 



The general views I hold with regard to the nucleus are stated 

 on p. 32 and I need not repeat them here. I only desire to 

 point out that the opposite view, viz. that the nucleus is iso- 

 lated so far as continuity of protoplasm is concerned, is, from a 

 physiological point of view, very difficult to accept ; and I think 

 that the burden of proof rests with him who maintains it. 



The peculiar lobed structure (PI. Ill, fig. 3) of certain stages 

 of the nucleus has been described before by other observers, 

 notably by Balfour in his " Monograph on the Development of 

 Elasmobranch Fishes,^' in the early stages of development. 



Klein in his communication on this subject, refers (10, p. 175) 

 to and confirms Strieker's (16) observations on the contractility 

 of the nuclear spongework and its continuity with the extra- 

 nuclear spongework in the colourless blood-corpuscles of the 

 newt and frog. He further confirms Strieker's statement as to 

 the disappearance of the nuclear membrane, and himself adds : 

 "The nucleus is therefore a part of the cell sub- 

 stance specially differentiated by the presence of a 

 membrane." Presumably Dr. Klein would still speak of a 

 nucleus when the membrane is absent. I am not able to make 

 out Klein's views with regard to this membrane. He says (11, 

 p. 415) : " In the convolution and basket of daughter nuclei the 

 membrane is very indistinct and is also here due to the close 

 position of the fibrils." I infer from this that he regards the 

 nuclear membrane as a part of the general reticulum at the 

 junction of the nuclear and extra-nuclear parts of the reticulum, 

 which gets in certain stages of the nucleus a regular ar- 



