108 PROCEEDINGS OF THE AMERICAN ACADEMY 



In eggs which have been fecundated artificially (Fig. 1), the sper- 

 matic particles surround like a halo the whole of the outer envelope, 

 beating about its surface with the greatest violence. Tlie yolk soon 

 begins to contract after the germinative vesicle and dot have dis- 

 appeared, and then divides into two spheres. (Fig. 2.) 



The segmentation takes place very rapidly, and as soon as there are 

 eight spheres (Fig. 3), they arrange themselves in such a manner as 

 to enclose the remaining space, which is more and more separated as 

 the spheres become more numerous ; finally, there is a complete en- 

 velope formed before the young makes its escape from the egg. 



The young when it escapes is spherical. The walls of the envelope 

 are of the same thickness. One side becomes thicker (Fig. 4) ; the 

 embryo flattens on the thick side. This wall is then bent in so as to 

 form a slight cavity, in which fluid circulates. (Fig. 5.) This cavity 

 extends half the length of the larva (Fig. 6), then swells at the ex- 

 tremities. The walls become thinner ; the pouch formed at the end. 

 of the cavity develops laterally to form two smaller pouches (Fig. 7), 

 which soon become small hollow bodies entirely separated from the 

 main cavity. (See Fig. 10.) The main cavity bends slightly to one 

 side (Fig. 8), and eventually forms a junction with a depression oppo- 

 site to it, and there the mouth is formed. The other opening, which 

 was the first to be developed, thus becomes the anus. 



This bent tube divides into three distinct regions, forming the oesoph- 

 agus, the digestive cavity, and the alimentary canal. (Fig. 9.) 



The small hollow bodies, the water-tubes, which are not connected 

 with one another in the young embryos (Fig. 10), differ one from the 

 other. One, the left (Avhen seen from above), connects with the sur- 

 rounding medium by means of an opening, the water-pore. (Fig. 9.) In 

 older specimens these two tubes extend to the extremity of the digestive 

 cavity, and towards one another, beyond the mouth, where they unite, 

 forming a Y-shaped tube. (Fig. 11.) Arms are developed which are 

 edged with rows of vibratile cilia. Some of these arms are of a differ- 

 ent character, having different appendages (see Figs. 12, 13). On 

 these water-tubes is developed the star-fish ; one of the water-tubes (the 

 one with the water-pore) developing the actinal side and the tentacles 

 (Fig. 13), the other developing the spines and the abactinal area (Fig. 

 12). These opposite parts of the star-fish are open (Fig. 13) pentagonal 

 spiral surfaces, not in the same plane, but making nearly a right-angle 

 with one another. The water-pore becomes the madrepoi'ic body. 



