TROPIOMETRA CARINATA. 7 



While in Antedon the eggs remain attached to the pinnulae during the 

 first stages of development, for a period of 4 to 6 days, in Tropiometra they 

 are attached only for a very short time. Almost immediately after the 

 extrusion from the genital opening the follicular membrane dissolves and 

 the naked eggs sink to the bottom; the fertilization does not take place until after 

 the egg has become free. Repeatedly I have found the bottom of the jar in 

 which the specimens were kept entirely covered with eggs discharged during 

 the night. Eggs were never found to be discharged during day-time. 



The formation of the egg-membrane is very interesting to follow. Immedi- 

 ately on the entrance of the spermatozoon the egg secretes a thick layer of a 

 slimy-looking substance, not regularly hmited outwardly. The inner part 

 of this layer at once acquires a harder consistency, thus forming a membrane, 

 the edge of which is sharply defined towards the egg-surface, while out- 

 wardly it acquires its final structure only gradually. The formation of 

 this structure starts at one place and spreads 

 from there over the whole egg (plate i, fig. 2). 

 When fully formed the membrane consists of 

 polygonal areas, shghtly sunken, with very 

 distinct, elevated edges. Each corner bears a 

 distinct spine (text-figure 1, «). The spines are 

 formed by the outer part of the slimy layer. 

 Sometimes I have observed a radiating stria- ^ ," t, x ,., 



. Fio. 1. — Part of the egg-membrane : 



tlOn m it. After the formation of the spines o, seen from above; 6, side view. X320. 



this layer is hardly discernible ; still, a fine line 



may be seen, uniting the points of the spines (text-figure l,b). The whole 



process occupies 15 to 20 minutes. 



The fully formed membrane (plate i, figures 3, 4, 5) is a very beautiful 

 object. It recalls the egg-membrane of Callionymus. I would suggest that 

 its peculiar structure is a special adaptation forming a floating apparatus. 

 Although the eggs were always found lying on the bottom in the dishes 

 until the embryo left the membrane, it can hardly be doubted that when 

 free in nature the sUghtest movement of the water must act on this spiny 

 membrane, causing the egg to drift. 



The egg-membrane of Antedon has received very little attention; only 

 Wyville Thomson describes it as "perfectly transparent and structureless, 

 with the surface slightly and irregularly echinated." (The Embryogcny of 

 Antedon rosaceus, p. 520.) In Antedon, therefore, there is evidently nothing 

 like the structure in Tropiometra. 



The segmentation begins very soon after the fertilization, the blastula 

 stage being reached after about 2 hours. In Antedon this stage is not reached 

 till 6 (Seeliger) to 12 hours (Bury) after the fertilization. The first cleavages 

 are quite regular (plate i, figures 3, 4, 5); in the later stages there is a slight 

 inequality, so that in the newly formed blastula the cells are somewhat 



