304 BULLETIN 82, UNITED STATES NATIONAL MUSEUM 



the structure at that time; but the fact that he did not notice anything of the kind 

 makes it fairly certain that no such structure of the follicular membrane exists in this 

 species. 



While in Antedon the eggs remain attached to the pinnules during the first stages 

 of development, for a period of from four to six days, in Tropiometra they are attached 

 for only 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. 

 Fertilization does not take place until after the egg has become free. 



Repeatedly he 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 daytime. 



The formation of the egg membrane, according to Mortensen, is very interesting 

 to follow. Immediately upon the entrance of the spermatozoon the egg secretes a 

 thick layer of a slimy looking substance, not regularly limited outwardly. The inner 

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

 the edge of which is sharply defined toward the egg surface, while outwardly it acquires 

 its final structure only gradually. The formation of this structure starts at one place 

 and spreads thence over the whole egg. When fully formed the membrane consists 

 of polygonal areas, slightly sunken, with very distinct elevated ridges. Each angle 

 bears a distinct spine. The spines are formed by the outer part of the slimy layer. 

 Sometimes he observed a radiating striation in it. After the formation of the spines 

 this layer is hardly discernible; still, a fine line may be seen uniting the points of the 

 spines. The whole process occupies 15 to 20 minutes. 



The fully formed membrane is a very beautiful object. It recalls the egg mem- 

 brane of Callionymus. He suggested 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 slightest movement of the water must act on this spiny 

 membrane, causing the egg to drift. 



Mortensen said that the egg membrane of Antedon has received very little atten- 

 tion. Only Sir Wyville Thomson describes it, as "perfectly transparent and structure- 

 less, with the surface slightly and irregularly echinated." In Antedon, therefore, 

 there is evidently nothing like the structure in Tropiometra. 



Segmentation begins very soon after fertilization, the blastula stage being reached 

 after about two hours. In Antedon this stage is not reached until 6 (Seeliger) to 12 

 (Bury) hours after fertilization. The first cleavages are quite regular. In the later 

 stages there is a slight inequality, so that in the newly formed blastula the cells are 

 somewhat larger at one pole of the embryo. This inequality, however, soon dis- 

 appears completely, and Mortensen was unable to ascertain whether the invagination 

 takes place at the pole where the larger cells occurred. 



When the embryo is about two and one-half hours old the formation of the 

 entoderm begins. It starts with the wandering into the cavity of the blastosphere of 

 several cells. These cells lie loosely in the cavity of the blastosphere and look like 

 mesenchyme cells which, however, they are not. It appears that they come irregu- 

 larly from different parts of the blastosphere. When the cavity is nearly full of these 

 cells the typical invagination takes place, and the loose cells now arrange themselves 



