388 Dr. E. IT. Henderson on the Development 



ventral surface, as the arms have not yet grown out and 

 differentiated themselves from the body, so that the embryo 

 from above appears spherical. In some instances (fig. D) 

 there could be seen distinct bosses, or projections, in each 

 radius along the line of, and just dorsal to, the tube-feet. 

 Tiiese are the arras of the future starfish. 



PI. XII. fig. E (ventral surface) shows the latest stage that 

 was found, and it existed in the material in great numbers. 

 The pentamerous arrangement is distinctly seen both from 

 above and below. The ridges shown in figs. C and D have 

 develojjed well-marked tube-feet in fig. E. These can be 

 counted by a hand-lens and appear as small round white 

 papillae, ten or twelve in each arm. At this stage the preoral 

 lobe is to a considerable extent atrophied, and is about to 

 separate off from the young starfish. 



I might here state that the adherence of the preoral lobes 

 to each other is due rather to some agglutinative material, 

 Avhich forms a coagulum between them, than to any cupping 

 or suction action, and that i\iey cling to one another thus 

 till the stalk is excessively atrophied and ready to separate 

 from the body. 



Having described the external appearance, I may now 

 briefly indicate the results of the study of sections. 



Methods adopted. — The great difficulty was to get these 

 large yolky larvae penetrated to the centre with celloidin or 

 paraffin, so that proper sections might be made. Both the 

 chloroform-paraffin and the benzol-paraffin methods were 

 tried, but in each case it was found to be impossible, even 

 with the greatest care, to get the paraffin to penetrate the 

 mass of yolk ; when the larvae were sectioned the centre 

 would drop out, leaving the outside tissue in the form of 

 a ring, and so the celloidin method had to be adopted. 

 Thin, medium, and thick solutions were used, since the more 

 gradual the change of density in successive solutions the 

 better is the degree of penetration. It is well to leave 

 them in each solution at least twenty-four hours, and a longer 

 time gives better results. Some of my best sections were 

 made last summer from material that had been lying in a 

 thin solution of celloidin for over a year. 



The sections from a celloidin-paraffin block have a tendency 

 to curl, a trouble which is remedied in a large measure by 

 having an exceedingly sharp razor and a Bunsen flame near 

 by or else bright sunshine playing over the surface of the 

 microtome. 



The sections were cut in varying thickness from 5 /w- to 10 /*. 



