The Microscope. 27 



the nucleus of segmentation, as described in the preceding sec- 

 tion, this compound structure occupies a place more or less 

 removed from the surface of the egg. In those forms where the 

 egg is nearly or entirely formed of protoplasm, or where the 

 protoplasm and food yolk are evenly distributed, the nucleus 

 occupies a position near the centre of the egg ; but when the 

 protoplasm is aggregated at one pole and the deutoplasm at the 

 other, the segmentation nucleus is found in the protoplasmic 

 portion, or," as it is called on account of the active part it plays 

 in the formation of the embryo, the animal pole, in contradis- 

 tinction to the deutoplasmic or vegetative pole. 



The egg is, as we have seen, but a single cell, the adult 

 animal which is to come from it is composed of myriads of cells. 

 How do the many arise from the one ? Simply by a process of 

 division, which theoretically takes place in a geometrical pro- 

 gression, though in reality the regular series is interrupted at 

 an early date. Theoretically, the series would be 1, 2, 4, 8, 16, 

 32, 64, 128, etc., but in reality this mathematical regularity is 

 not the rule, and frequently the divisions are not completed 

 until a very late stage. The cause of this variation and irregu- 

 larity is not far to seek. As we have seen, each egg is made up 

 of protoplasm and deutoplasm variously distributed, and the 

 relations of these influence the rapidity and the character of 

 the segmentation. The protoplasm is the active part; the deu- 

 toplasm, on the other hand, is inert and restraining. So when 

 we have an egg with the food yolk accumulated at the centre, 

 the surface divides first, and the deeper portions of the cell 

 walls do not appear until a later stage. Where the protoplasm 

 accumulates at one pole, that is the first to segment, and the 

 planes ol division do not appear at the opposite one until after 

 some time has elapsed. The first of these types may be seen by 

 studying the egg of most of the Crustacea, while a good exam- 

 ple of the other will be found in the segmenting egg of a frog. 



In most fishes, especially those with floating eggs, the pro- 

 toplasm and the food yolk are almost completely separated, and 

 the former alone segments, forming a little cap of cells resting 

 upon the deutoplasm, which is thoroughly inert. 



Recent investigations tend to show that the first segmenta- 

 tion plane coincides with the axis of the future embryo, one of 



