306 



ALLEN. 



[VOL. I. 



the rays at first being very short. These processes elongate 

 to form the basal tentacles of the young hydroid. While this 

 growth is taking place the convex side of the embryo becomes 

 still more convex, and the concave portion between the tenta- 

 cles evaginates and becomes convex also. In this way the 

 endoderm cells in the center are split apart and the body cavity 

 is formed. At first it is very irregular, but later the endoderm. 

 cells assume the typical endodermal form and arrange them- 

 selves in a single layer within the ectoderm, and the body cavity 



FIG. 3. 



FIG. 4. 



FIG. 3 x 560. Fully formed ectoderm from convex side of embryo; ec, ectoderm ; en, endo- 

 derm ; n, nuclei ; a, nuclei of absorbed eggs. 



FIG. 4 x IQO. Young embryo ready to escape ; t 1 , basal tentacle ; t-, buccal tentacle ; s, stalk ; 

 cc, ectoderm ; en, endoderm. 



takes on a form quite similar to that of a young polyp. From 

 almost the earliest stage in the development of the ectoderm 

 the cells on the convex side of the embryo appear much longer 

 than upon the opposite side, and it is this portion which be- 

 comes the stem to which the young hydroid attaches itself. 

 According to Agassiz the larva escapes in this condition, and 

 the mouth and buccal tentacles are developed after it attaches 

 itself. I have, however, obtained sections of a large number 

 of mature larvae in which well-developed tentacles were pres- 

 ent. Fig. 4 represents an embryo that was just leaving the 

 gonophore. The body and the stem were both well developed, 

 and the basal tentacles were nearly twice as long as the body. 



