330 Prof. E. W. MacBride. The Artificial Production of 



of the anus. These latter are termed post-oral arms. The two arms on 

 each side of the larva are supported by delicate calcareous rods which are 

 outgrowths of a calcareous star, the beginnings of which can be traced back 

 to the blastula stage. On the 5th day the left ccelomic sac sends out a 

 tubular protrusion, which ascends to the dorsal surface, fuses with the 

 ectoderm, and opens to the outside. This outgrowth is the pore-canal, 

 and its opening to the exterior is the primary madreporic pore. 



The development up to the point which I have just described is exceed- 

 ingly familar to all workers in experimental embryology, for Echinus miliaris 

 is closely allied to the famous Echinus viicrotuherculatns of the Bay of Naples, 

 if indeed it be not merely a northern race of that species. As all know, 

 countless experiments have been made on the larva of Echinus microtuiercu- 

 latus, hut few experimenters have carried their cultures of the larvae beyond 

 the point of development which has just been described, which is just the 

 point at which the larvae begin to require food. 



As mentioned above, the appearance of the older larva was first described 

 by me (12) and the only series of experiments in which the older larva has 

 been used are those carried out by Debaisieux (3) in the laboratory of the 

 Imperial College under my supervision and those made in the marine 

 biological laboratory at Plymouth by Shearer, De Morgan and Fuchs (16). 



When the larva has attained the age of about eight days the left coelomic 

 sac becomes divided into anterior and posterior portions, and a little later the 

 right coelomic sac begins to be constricted into anterior and posterior portions 

 (fig. 2), but these parts are not finally divided from one another until the 

 12th day. On the 10th day the hydrocoele begins to be formed as a 

 swelling of the hinder end of the left anterior ccelemic sac. This bud-like 

 swelling is never completely separated from the rest of the sac ; the neck of 

 union between the two portions becomes drawn out so as to form a narrow 

 tube, which is the stone canal {st.c, fig. 4). It follows that the coimexion of 

 the hydrocele with the exterior is an indirect one, for the left anterior coelom 

 persists into the adult stage as a sac-like space termed the axial sinilS, and the 

 stone canal leads from the hydrocoele or umter vascular ring into this sinus, and 

 from this sinus the pore-canal (which in the adult hecomes multiplied into 

 numerous pore-canals) leads to the exterior. 



The ciliated band grows out on each side into an extra arm termed the 

 postero-dorsal arm, which is supported by a calcareous rod growing out 

 from a new calcareous star on each side. In the bay surrounded by the loop 

 of the ciliated band leading from the base of this arm to the base of the post- 

 oral arm, an invagination of the ectoderm takes place which is termed the 

 amniotic invagination {am., fig. 3). The ectoderm at the bottom of 



