THE PRESENT POSITION OF CELL-THEORY. 315 



growing tissues of many animals and plants, as for instance 

 in the embryoes of many Arthropods, in the endosperm of 

 Phanerogams, etc. The condition may be permanent, as 

 in the case of the Cceloblastae, or non-permanent, as in the 

 other cases. But in both instances there is a difference 

 from the ccenocytial condition observed in Protozoa, namely, 

 that the multiplication of the nucleus does not lead to re- 

 production in the form of the splitting up of the biont into 

 as many new bionts as there are nuclei. 



In a ccenocytial biont of appreciable size the relations 

 of the various parts to external conditions will tend to be- 

 come different, and differences of chemical constitution will 

 be set up in the different regions exposed to different con- 

 ditions. We can see that this is the case in Botrydium, in 

 which root and shoot are plainly marked off from one 

 another, and better still in Caulerpa, in Codium, and in 

 many of the Moulds. Differences in chemical constitution 

 thus induced will mean difference in exchange between 

 nucleus and cytoplasm, and we may infer that, in accordance 

 with these differences, the cytoplasm within the limit of 

 influence of any one nucleus will in time assume a con- 

 stitution so different from that of the adjacent cytoplasm as 

 to become sharply marked off from it. It will then acquire 

 its own surface tension — the first step towards a cell wall— 

 and will be a separate corpuscle containing a nucleus, in 

 fact a cell. Such a cell however has not come into being as 

 an individual unit joined to its like, either phylogenetically 

 or ontogenetically, but it has from the first formed a part 

 of an organic whole, of which it is nothing more than a 

 specialised component part. 



One looks naturally for evidence of this mode of forma- 

 tion of cellular structure in developing Metazoa. The best 

 evidence is to be found, I think, in the segmentation and 

 formation of the layers in many Ccelenterata. In some 

 Ccelenterata — for example, in Renilla — the nucleus divides 

 without accompanying division of the cytoplasm until eight 

 or sixteen nuclei are present, and then the cytoplasm 

 divides and eight or sixteen cells are formed. But of more 

 importance than this is the formation of the layers. From 



