258 AYERS ON THE DEVELOPMENT 



appear in the process of segmentation. There exists this striking difference between the 

 two cases, that whereas in most cases the nucleoli (polar corpuscles) are placed in the cell 

 protoplasm, in the case of the serosa nuclei the corpuscle lies within the membrane of the 

 nucleus, and the division of the latter does not at first seem to affect the condition of 

 the cell. 1 (PI. 20 figs. 30, 31.) Such a cell immediately after the division of the nucleus is 

 practically in the condition of a syncytium. The relations of the nucleolus to its fibres and to 

 the nuclear fluid are distinctly shown in sections of endodermic nuclei. (PI. 21, fio- s . 5, 11, 12.) 



As regards the origin and significance of the primitive germinal, layers among the 

 Insecta, I am still in doubt. Balfour's interpretations and general conclusions (loc. cit., 

 Vol. I, p. 378 ; Vol. II, p. 278) I cannot accept since they seem at variance with the facts. 

 The author considers the superficial cell layer existing at the close of the formation of 

 the blastoderm equivalent to ectoderm (epiblast) and the inclosed yolk mass as essentially 

 endoderm. But when we consider the role and subsequent fate of these two layers it at 

 once becomes apparent that such a view does not accord with the facts, for the blastoderm 

 ultimately forms the entoderm (mesenteron), and the ectoderm, arising from a small area 

 of thickened cells on one side of the blastoderm, encloses the yolk and endoderm by a 

 genuine epiboly. 2 



It is, however, with some hesitancy that I expose the following tentative views on the 

 subject, with the hope that, if they do not at present afford an entirely satisfactory and 

 complete explanation of the facts, they may at least help toward the solution of the diffi- 

 cult problems of the origin and significance of the embryonic membranes and the germinal 

 layers in the Arthropoda, and in this way serve, perhaps, to throw some light on the phy- 

 logeny of the group. 



How did the embryonic membranes (amnion and serosa) arise ? What is their function ? 

 Is their present function the primitive one ? The answer to these inquiries undoubtedly 



lies in the clear comprehension of the relations of the embryo to its food supply the 



yolk. That the cellular embryonic membranes could have originated for protection, or 

 from an early ecdysis is, to say the least, highly improbable. Among the Insecta the egg 

 is furnished with a protective membrane — usually in the form or a chorion before leav- 

 ing the body of the parent. The presence of an unusually large amount or yolk matter, 

 fundamentally changes the manner of cleavage in the egg of the Arthropod. 3 After the forma- 

 tion of the blastoderm the yolk is inclosed in a cellular membrane which is, strictly speaking, 

 a food sac or stomach ; functionally considered it is consequently endodermic, not ectodermic, 

 in its nature. The ectoderm arises, as has been said, in a limited area on one side of the 



« In most animals the polar corpuscles arc formed just at stance (e. g. most animals and plants) ; or they may lie at a 



the outer edge of the nuclear membrane and exert an attrac- distance outside the membrane and influence both nuclear 



tion in all directions, through the nuclear substance, pro. and cell protoplasm (e. g. Liuiax; see Literature, 30). Since 



during the spindle phenomenon and through the yolk, produc- these polar corpuscles are essentially nuclear in their origin 



ing the astral structures. In some cases (e. g. Limax) they the last two cases are to be considered as derivatives of a 



are formed at a distance outside the nuclear membrane and primitive form, in which the corpuscles are internn 



iiiclear. 



never come in contact with it. There are, then, three posi- * The so-called endoderm, on the other hand, remains 



tions which the polar corpuscles may assume. They may lie throughout its existence an inert mass of food substance 



entirely wrthm the nuclear membrane, the latter apparently between the particles of which numerous indifferent cells are 



setting a hunt to its influence (e. g. Oeeanthus); they may found. 



lie in contact with the outer surface of the nuclear membrane a In Oeeanthus there is no trace of yolk segmentation. 

 and exert their influence through both nuclear and cell sub- 



