766 PROCEEDINGS OF THE AMERICAN ACADEMY. 



In no part of the axis of a stem or branch has the marrow grown in 

 opposite directions, part distall}', part proximally, for the walls of the 

 chambers are always convex toward one end of the stem or branch — 

 the distal end (Plate 4, Figs. 57, 61). The marrow must therefore 

 have grown from the base of the branch toward the tip, just as it does 

 in the stem from base toward tip, and not, as maintained by some 

 writers, in the form of a separately established axis which grows in 

 two directions : partly toward the tip and partly toward the stem .to 

 which, in their view, it is destined to be attached secondarily. 



This type of axis — with the axis-cortex interposed between tlie 

 marrow chambers of stem and branch — is the natural one for all 

 axes except such as may have been formed by the dichotomous branch- 

 ing of a main stem. Such branches may possibly occur, but my 

 dissections have not shown any. Occasionally, in colonies that had 

 attained a height of ninety or more centimeters, the beginning of a 

 branch was found on some of the whips. These, as short as five milli- 

 meters, had a soft axis that was continuous with the main axis and 

 was formed of the characteristic marrow (Figs. 57, 61, mod. ax.) and a 

 very thin cortex. The marrow chambers were separated from those 

 of the main stem by the cortex region of the stem-axis and, as has 

 already been stated, were convex toward the free end of the branch, 

 as in the main branch they were convex toward its free end. Although 

 the earliest stage in the formation of the axis of a branch has not been 

 seen in Pseudoplexaura, I am convinced that the axis skeleton of the 

 whole colony in this species is not produced by a coalescence of sepa- 

 rately established axes. 



Pseudoplexaura gives no evidence on the third of Schneider's points, 

 for no horny substance has been found in the coenenchyma. But the 

 real issue between the two theories of ectodermal or non-ectodermal 

 origin hinges on the results of observation as to the origin of the axis 

 and as to the method of its subsequent growth; whether it is an epi- 

 thelial secretion, as argued by von Koch ('78, '87), or results from a 

 massing of mesogloeal material which is to be resorbed and replaced 

 either by horny substance or horn and lime. Von Koch has described 

 ('87) a larval stage of Eunicella a week old and has shown sections 

 having the ectoderm continuous with the axis epithelium. His results 

 have recently been confirmed by Kinoshita (:10) in embryos of Antho- 

 plexaura. Kinoshita not only found the ectoderm of the pedal disk 

 continuous with the axis epithelium, but he also has described and 

 figured (Fig. 3-5) the beginning of the axis as a secretion product 

 of the thickened ectoderm of the pedal disk; however, this primitive 



