HISTOLOGY AND DEVELOPMENT OF MTRIOTHELA PHRYGIA. 507 



The Early Stages in the Formation of the Gonophore 

 AND the Origin of the Sexual Elements. 



The points noticed under this heading may be regarded as 

 an addendum to Korotneff's account of the development of the 

 gonophore (3) alluded to above. His brief and somewhat 

 diagrammatic account is illustrated by figures which unfortu- 

 nately show little attempt at histological detail. 



The Structure of the Ectoderm of the Blasto- 

 style. — In each blastostyle two regions may be distinguished ; 

 a distal and shorter region bearing several small capitate ten- 

 tacles, and a proximal region, on the whole free from tentacles, 

 on which the gonophores are developed. The latter embraces 

 two-thirds to four-fifths of the whole length of the blasto- 

 style. The ectoderm of the distal portion resembles that of 

 the body generally, with the exception that the muscle-fibres, 

 which form its deepest layer, are not so strongly developed. 

 Its structure is shown in PI. XXXVI, fig. 1. Like the ectoderm 

 of the animal generally, it is covered superficially by a well- 

 marked cuticle, which, when stripped off and examined with a 

 high power, is seen to be divided into irregular areas, doubtless 

 corresponding to the subjacent cell layer. This latter is com- 

 posed of long columnar cells, tapering at their lower end. Each 

 cell is about 25 fx long, though their dimensions possibly vary 

 according as the whole blastostyle is in a condition of exten- 

 sion or contraction. Lying here and there between the pointed 

 bases of these cells are scattered ganglion-cells, which are con- 

 nected with a rich plexus of nerve-fibrils situated in the deepest 

 part of the ectoderm and immediately over the muscle-fibres. 

 The latter run longitudinally, and immediately overlie the sup- 

 porting lamella. Fibrils from the basal nerve plexus pass 

 between the columnar cells towards the surface, and, in tangen- 

 tial sections, are seen to form a superficial plexus between the 

 columnar cells. 



The same type of nucleus is found throughout the whole 

 ectoderm. It is characterised by the presence of a distinct 



