308 W. WOODLAND. 
thus fixed and preserved in absolute alcohol, were then simply 
packed in small tubes duly labelled until it was convenient to 
examine them. I found that it was of no use adopting the 
method of preparation usually employed in the study of spicule 
formation, viz. fixing the tissues with osmic acid and staining 
with picro-carmine, since these reagents destroyed the skeleton 
within a very short time. Indeed, like the spicules of most 
calcareous sponges, the pluteus skeleton is very susceptible 
to traces of acid, and the utmost care must be taken to exactly 
neutralize all reagents employed in its preparation. The larve 
preserved in neutral absolute alcohol were doubly stained 
with safranin (the larve remaining for at least a week in a 
saturated solution in absolute alcohol) and nigrosin (the larve 
remaining half an hour or more in a similarly saturated solu- 
tion), though, judging from my later experience in preparing 
holothurian spicules, I have no doubt but that licht grin 
(about fifteen minutes’ immersion in the same saturated solu- 
tion) would form a satisfactory substitute for the latter stain. 
The result was that the nuclei assumed a pink colour and 
contrasted well with the slaty hue of the cytoplasm. 
Before proceeding to the subject proper of the present paper 
I wish to express my warmest thanks to Dr. EH. J. Allen, who 
very kindly afforded me all facilities for carrying out my work 
at Plymouth, to Prof. Minchin for general advice, and to the 
Council of the British Association who, at the kind suggestion 
of Mr. Garstang, granted me a free table. 
THe ORIGIN OF THE SPICULES IN THE PLUTEUS OF 
EK. ESCULENTUS. 
As is well known, the mesenchyme cells, the great majority 
of which are skeletogenous in function, arise in the blastula 
(text-fig. 1) of E. esculentus, as in Echinoderm blastule 
generally, by the migration into the blastoccele of cells at first 
situated in the flattened posterior wall of the larva (text-fig. 2), 
and later at the convexity of the hypoblastic invagination (text- 
