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Mr. A. Sedgwick. On the Fertilised [Nov. 26, 



2. All the cells of the ovum, ectodermal as well as endodermal, are 

 connected together by a fine protoplasmic reticulum which is 

 placed, as are also the cells, immediately beneath the egg membrane, 

 and therefore around a central space. 



Each ectoderm cell consists of a central nucleus around which is a 

 close protoplasmic spongework, which at the outer parts of the so- 

 called cell becomes of a gradually looser nature until it runs into the 

 spongework of the surrounding cells. 



Each endoderm mass consists of a central denser spongework which 

 gradually becomes looser towards the periphery of the mass until it is 

 continued into a fine reticulum. The endoderm masses are far apart 

 from each other and are connected by this reticulum. 



The continuity of the various cells of the segmenting ovum is 

 primary and not secondary, i.e., in the cleavage, the segments do not 

 completely separate from one another. But are we justified in speak- 

 ing of cells at all in this case ? The fully segmented -ovum is a 

 syncytium, and there are not and have not teen at any stage cell limits. 

 I think the cleavage should be rather described not as segmentation, 

 but a multiplication of the nucleus or centre of force which causes a 

 corresponding readjustment in the density of the network at different 

 parts of the ovum, but no break in continuity. 



The Structure of the Gastrula. 



The formation of the gastrula, which is at first solid, has been 

 described in my first communication to the Society on this subject. 



The endoderm masses at first, as I have already mentioned, have no 

 nuclei. Nuclei first appear in them during the progress of the epibole 

 by which the gastrula is formed. I have not been able to determine 

 the origin of these nuclei. They either arise de novo in the endoderm 

 masses or migrate into the latter from the ectoderm. The proto- 

 plasmic network at the centre of each endoderm mass is denser than 

 at the periphery, but is without the chromatin granules, so charac- 

 teristic of a nucleus. But I have already described a stage of the 

 nucleus in the fertilised unsegmented ovum in which the chromatin 

 granules are almost entirely absent, and in which the network pre- 

 sents no essential difference from the surrounding network. Again, 

 another in which the nuclear network merges so gradually into the 

 surrounding network, that it is impossible to point to any limit be- 

 tween them. I therefore think it quite possible that this central 

 denser protoplasm in the endoderm masses may give rise to the 

 nucleus which subsequently appears. But on the other hand I must 

 distinctly state that this is only a possible view which may be borne 

 in mind, but which cannot be accepted without the most overwhelm- 

 ing proof in the present state of our science. 



The solid gastrula is a syncytium ; the ectodermal nuclei are 



