THE RELATIONSHIPS OF THE SEA-SPIDERS. I47 



there are two types of development within the group, 

 and only to one of these have I paid, so far, special 

 attention. 



The first type is that represented by the genus Tany- 

 stylum, and is to be regarded as the more primitive of 

 the two, which I shall describe. 



The egg of Tanystylum undergoes a regular equal 

 segmentation into two, four, eight, etc., resulting in 

 a solid mass of cells, each cell pyramidal in shape, 

 with its apex at the centre of the egg, and the base 

 at the surface. Every cell contains a single nucleus 

 lying nearer to its base ; that is, to the outer part of 

 its cell. 



Each of these cells then divides — the nucleus dividing 

 into two at the same time — into an inner and outer half, 

 so that there results a solid two-layered embryo. This 

 change has been effected by a process of multipolar 

 delamination. The inner cells, or at least some of 

 them, go to form the endoderm of the adult ; the outer 

 form the ectoderm. 



I have not followed in detail the changes which take 

 place after this, so that we must pass to a stage of 

 development when the young embryo has broken 

 through the egg membranes and has begun to lead an 

 independent life. At this stage it is totally different 

 from the adult, although, as we shall see later, it con- 

 tains many of the essential characteristics of those parts 

 of the adult which it represents. Figure 2 gives an out- 

 line of a larva at such a stage. We see it has three 

 pairs of appendages, of which the first is chelate, and 

 the second and third pairs are simple uniramous struc- 

 tures, which are used in crawling around amongst the 



