THE PYCNOGONIDS. 5 



of an egg of Tanystylurn immediately after del animation has 

 taken place. 



Exactly similar changes were seen in the eggs of Phoxichi- 

 lidium, but the figures just given serve in every respect for both 

 species. The central as well as the peripheral cells continue to 

 divide, but soon the entoblast cells lose their well-defined 

 boundaries and the nuclei seem in part to disappear. The result 

 is that we have in the center of each egg a granular yolk with 

 scattered nuclei in it. Here and there is an indication of a cell 

 boundary. During this time the ectoblast cells have divided 

 tangentially and have become much smaller, yet at all times a 

 distinct boundary remains between ectoblast and entoblast. Fig. 

 12 for Phoxichilidium shows an embryo at this stage. 



It is here seen that the ectoblast cells over one hemisphere are 

 somewhat higher than at the opposite, and I find this very con- 

 stant in sections of both species at this stage of development. 

 After this it becomes difficult to follow out the fate of the germ- 

 layers. The outer cells become smaller and flatter to form the 

 ectoblast and the inner cells arranged into organs, the most con- 

 spicuous of which is the digestive tract. There is a triangular 

 invagination to form the stomodseum, and the proboscis appears 

 between the first pair of appendages, which has now begun to 

 form. These appendages are very conspicuous in surface views, 

 where they project beyond the surface of the body. Between 

 them appears the slightly projecting proboscis, and about the 

 middle of the embryo are seen the second and third pairs of 

 appendages, which are small and inconspicuous. Dohrn has 

 given excellent figures of embryos at this stage, both in his 

 earlier paper and in his later monograph. Soon after this the 

 egg coverings swell up somewhat and the embryos finally break 

 out of the egg case, so that the appendages now can straighten 

 out. 



In Plate IY, Fig. 9, is a surface view of a larva of Tanystylum 

 as seen from the ventral surface. Dohrn speaks of this as the 

 Pantopod-larva, and Hoek calls it a Protonymphon. Its general 

 characters are shown in the figure. There are three pairs of 

 appendages : the first pair chelate, with the movable daw working 

 outwards and downwards ; the second and third pairs have 

 distally a sharp spine, in the middle of the limb a large segment, 



