102 OTTO F. KAMPMEIER 
form, in so far as their nuclei appear compressed and uniformly 
dense, but others again contain large, spherical, and coarsely 
chromatic nuclei and protrude into the heart cavity like little 
humps or hillocks, which call to mind the observations of certain 
investigators on haemopoiesis where endothelium germinated 
blood cells, but the author was unable to discover an undoubted 
case where one became constricted off. 
The further differentiation and thickening of the heart wall 
occurs during the period of growth after metamorphosis, and 
histological examination of a section through the lymph heart of 
the adult anuran reveals three well-defined coats or layers: a 
tunica interna or intima, a tunica media, and a tunica externa 
or adventitia. The first is composed of the layer of highly 
flattened lining cells and a very thin stratum of connective tissue, 
probably elastic in nature, immediately external to them. As we 
should expect from the great energy displayed by the lymph 
hearts during life, the muscular tunica media, the second coat, 
is the broadest layer of the heart. Its muscle cells or fibers are 
of varying length and thickness and group themselves into small 
bundles which branch and interlace in a complex manner. Hoyer 
(704) claims that the individual muscle fibers themselves branch 
and anastomose and possess numerous cross bands, which call to 
mind the intercalated dises of human cardiac muscle. A large 
number of elastic strands are also contained in the media. No 
sharp boundaries exist between media and adventitia. ‘The 
latter is made up of fibrillar connective tissue in which are seat- 
tered pigment cells. The nerve fibers to the anterior pair of 
lymph hearts are apparently supplied by the III spinal nerve. 
According to Waldeyer (’64), both medullated and non-medul- 
lated nerve fibers are found in the walls of the fully developed 
lymph hearts. 
Before discussing the formation of the valves, a variable feature 
may be mentioned in connection with the development of the 
walls. In about half of the lymph hearts examined between 8- 
and 16-mm. stages, a strand or trabecula, sometimes delicate 
and sometimes fairly thick, bridged the cavity. Occasionally 
they were imperfect, simply projecting as slender filaments 
