THE ENDODEEMAL NUCLEI. 35 



I have no observations on tlie transformation of the simple 

 male pronucleus of the early stages into this complicated 

 structure, nor have I any on the transformation, quite as 

 remarkable, of the few chromatin masses, which represented 

 the female pronucleus in the last described ovum, into the 

 complicated structure present in this case. 



3. The Nucleus of the Ectoderm in the gastrula 

 and later stages. — I have already (p. 33) said all that I at 

 present have to say about this nucleus. It is much smaller 

 than the earlier nuclei, and not specially favorable for study. 

 I have little doubt, however, that the network of which it is 

 composed is continuous with the external spongework. 



4. The Endodermal Nuclei. — As I have already said 

 there are apparently no nuclei in the endoderm masses of the 

 segmenting ovum, or, in other words, no part of the first seg- 

 mentation nucleus enters, so far as I could see, these masses 

 during the segmentation. At any rate there can be, I think, 

 but little doubt on one point, viz. that the endoderm masses 

 do not during the segmentation contain any structure like a 

 nucleus as ordinarily described. They do contain, as I have 

 already said, a densely I'eticulated central area, but this is with- 

 out any deeply-staining chromatin so characteristic of a nucleus. 

 Can this area represent a nucleus, perform the functions of a 

 nucleus for these endodermal masses ? 



Without venturing to decide the question I may draw atten- 

 tion to two facts brought out by the study of the large nuclei 

 described under heading 1 (p. 28). These are: (1) The 

 nuclear spongework is perfectly continuous with the extra- 

 nuclear spongework, and (2) the amount of concentrated deeply- 

 staining matter may be very small, as in the undoubted nucleus 

 of fig. 2 in which the three masses in the figure represented 

 the whole of the especially deeply-staining matter present. 



The question, therefore, presents itself ; what is the essential 

 part of the nucleus ? Is it the spongework or is it the deeply- 

 staining parts of the spongework ? A comparison of figs. 2 and 

 3, PI. Ill, in which the amount of deeply-staining matter is so 

 different, favours the first view, viz. that the essential part of 



