452 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



in the inner cell end near the nucleus, sometimes on the outer side in part extruded 

 from the cell. These inclusions appear to be products of the cells in which they 

 lie, and the cells appear to be ectodermal gland cells. The function of the cells 

 and of their secretion is unknown. 



During the free-swimming larval period the cells excrete externally a fine 

 cuticular membrane, and between them a clear homogeneous transparent glass- 

 like intermediate substance appears, first at the outer cell ends, later advancing 

 gradually inward. Simultaneously the nuclei move into the inner ends of the 

 cells and their chromatin content again increases. 



Very soon after the commencement of the secretion of the intermediate sub- 

 stance the cells begin to withdraw from the surface, becoming club-shaped, and 

 occasionally almost spherical, and lying at the base of the ectoderm between 

 the protoplasmic bodies of the other cells which still reach to the outer membrane, 

 but which are no longer throughout parallel to each other. 



To what extent the shortening of the cells is related to the secretion of the 

 intermediate substance is not clear, but the rounded cells have lost in volume, 

 while among the elongated cells many can be seen which are about as large as 

 in the first few hours of free existence. 



Toward the body cavity the border of the ectodermal layer is no longer so 

 well defined as in the younger stages, for the club-shaped and rounded cells 

 project more or less inwardly; but in spite of this and of the fact that the 

 formation of the intermediate substance gives the ectoderm a certain similarity 

 to connective tissue, especially in later stages, no doubt can at any time exist during 

 the free-swimming period as to whether a given cell belongs to the ectoderm or 

 to the mesoderm. 



Apical pit and nervous system. During the free-swimming larval period the 

 apical pit and the subjacent nervous layer reaches its highest development, only 

 to dwindle and disappear soon after fixation. 



In the preceding stage individual ectodermal cells had migrated to a position 

 beneath the ectoderm where, separated from the body cavity by a membrane, they 

 had changed their form and a fine fibrous layer had appeared. 



Little by little the number of these subepithelial cells increases, chiefly through 

 the inward migration of new ectoderm cells, but also by division. They become 

 markedly shorter, and send out one, two, or three processes. They become rounded 

 when about to divide. These are evidently bi- or multipolar ganglion cells. 



The size of the cell body is variable, between 0.005 mm. and 0.006 mm. 

 measured without the processes, which may be of various lengths. 



The cells lie on the fiber layer, between it and the ectoderm; a few of them 

 force themselves deeper and become entirely surrounded by the fibers. 



The fiber layer has become very considerable under the apical pit, and extends 

 thence outward over the anterior end of the larva, becoming gradually finer toward 

 the periphery and disappearing at the second ciliated bands. Only individual 

 fibers proceed further posteriorly along the sides to supply the posterior ciliated 

 bands. 



