LIVING CONNECTIVE TISSUE CELLS 131 



The viability of the animals is unaltered for I have kept them 

 for several days after such observation without any indication of 

 decreased activity on their part. Even embryos which have been 

 quieted by chlorotone, as well as those immersed for hours in a 

 solution of Bismark brown in sea water, I have resuscitated and 

 kept alive and in an apparently normal and usual condition for 

 two or three days; they could easily have been kept longer had it 

 seemed advisable. 



The tissue selected for observation was in the fins of free swim- 

 ming pelagic and Fundulus embryos. The embryos studied were 

 chiefly of Fundulus and varied from 5 to 20 mm. in total length. 

 The most favorable subjects were from the time of hatching, 5 

 to 6 mm., up to 12 mm. in length. The pectoral and caudal fins 

 were usually selected as most available for observation. In 

 such embryos the fin consists of a central frame work formed by 

 the jointed rays, lepidotrichia, with their attached muscles, and 

 a superficial integument of pavement epithelium with its sub- 

 jacent basement membrane. The finer fin rays, actinotrichia, 

 continue the jointed rays to the margin of the fin. The fin at 

 this stage is very thin and the epidermis lies almost in contact 

 with the fin rays. But between adjacent rays is an interval which 

 lodges on either side the afferent and efferent blood vessels, bor- 

 dered by chromatophores, and between them a loose mass of mes- 

 enchymal connective tissue in which the cells may be readily 

 observed. 



In embryos 5 to 6 mm. long the connective tissue in the pectoral 

 fins consists chiefly of a mass of round cells confined to the proxi- 

 mal portion, and beyond this mass a distal fringe or 'skirmish 

 line' of scattered stellate cells. In the unpaired fins, which are 

 less advanced in their development only the scattered stellate 

 cells are represented, the invasion of the round cell mass having 

 not yet occurred. In later stages, as in the caudal fin of the same 

 embryo, the zone of round cells has advanced distalward among 

 the actinotrichia nearly to the fin margin, leaving behind between 

 the lepidotrichia an area of more mature cells, stellate and spindle, 

 and a few fine fibers well separated by broad spaces occupied by 

 tissue fluids. The spindle cells and fibers preponderate in the 



