235 



Fig. 1. 



Stomach /- 



1< 



Coelom 



Fseudoheart 



Sinus a 



- Stone Canal 



discover this is the first time that a systematic study of the develop- 

 ment of this problematical organ has been made in any Echinoderm. 



The literature of the subject shall only be noticed in so far as my 

 results confirm those of other observers. 



Unlike some previous workers I have not founded conclusions on 

 sections of adult animals containing Embryos, since in this case not 

 only has one no control over the orientation of the young, but also the 

 preserving fluids often penetrate to them very imperfectly. 



In all cases the young were extracted from the mother and orien- 

 tated by breaking ofi" an arm whilst living so that when decalcified 

 and stained the position of the madreporic plate could be recognized. 

 Osmic acid was the harden- 

 ing reagent employed, since 

 the most perfect preserva- 

 tion especially as regards 

 cavities, is obtained by 

 this method. Double stain- 

 ing was also used in every 

 case. 



In embryos measuring 

 200 — 220 [JL in diameter in 

 which the arms are still 

 undeveloped, the coelom 

 is filled by a mass of me- 

 senchyme. Slightly later 

 its cavity is visible as a tri- 

 angular space at each side 

 of the stomach. Coverinsr 

 the external wall of the 

 stone-canal is a single layer 

 of nuclei, which may there- 

 fore be called peritoneal, 

 though they are more roun- 

 ded than the rest of the peritoneal nuclei. This layer is the 

 first rudiment of the pseudo-heart. Very soon, proliferation 

 can be seen in this layer. The nuclei increase in number, get 

 larger and one can distinguish round one or two of them a rounded 

 cell body. At this stage when the embryo has attained a diameter of 

 300 [x, the first rudiments of the axial sinus are visible. I have 

 been able to distinguish three separate rudiments and to trace their 

 development into the adult : all of these have been confounded by 

 most observers under the name of axial sinus. Ludwig in his «Neue 



Sums & 



Fig. 2. 



Sinus c 



Stomach- 



Coelom^ 



Sinus a- 

 Sinus b 



CcuiaL 



Simrs , 



