243 



the renal vesicle. The pericardium is closely attached to the wall of 

 the renal vesicle along its ventral edge, while the pericardial cavity 

 extends dorsally almost to the outer wall of the ovary. (See Fig. 1.) 

 The heart itself occupies the more ventral part of the paricardial 

 space, and in section is seen to He against the renal vesicle. 



Seen in cross - section the muscles of the heart may be said in 

 general to take a longitudinal direction, running spirally around the 

 heart. The muscle fibers when relaxed are quite regular in form, in 

 longitudinal section they appear to be almost losenge shaped. The 

 cells are from 60 /n to 90 /^i in length and from 3 f.i to 5 jli in width 

 at their widest part. The length varies greatly with the state of con- 

 traction. The muscle fibers appear to be uni- nucleate. The nuclei 

 are ovoid in shape, measure 3 /< by 5 /<, and contain usually from 

 2 to 3 pieces of chromatin. These chromatin masses are irregular in 

 shape, and lie close to the periphery of the nucleus. The fibrillar 

 portion of the cell is destinctly cross - striated ; the deeply staining 

 anisotropic substance forming groups of four disks, which under the 

 low power look like a single broad band. The muscle fibrils lie on 

 the inner side of the cell separated from the blood by the endocardial 

 lining. (See Fig. 2.) 



,e .n . 



Fig. 2. Tangential section of wall of heart, Molgula. Flemming: Iron-ha?mato- 

 xylin. Leitz, Oc. 3. X V'l« Imm- c«. ii. nuclei of connective tissue elements, e. n. 

 nuclei of endothelial cells, m. f. muscle fibers, m. n. nuclei of muscle cells. 



Lying closely applied to the muscle cells, and forming the outer 

 surface of the heart proper, are found the so-called "connective tissue 

 elements". These cells stain only after relatively long immersion in 

 raethylen blue. They encircle the heart in tolerably regular spiral 

 lines. ScHULTZE shows such cells as endocardial in nature. I have 

 also found cells reacting in this manner to methylen blue in a Salpa 

 (Yarsis cordiformis zonaria) and in Ciona intestinalis. The 



16* 



