58 THE BIOLOGY OF HYDRA : 1961 



simple bars but actually form lamellar partitions, or septa. The exact 

 nature of the septa is not yet clear but there is some indication 

 that they may be continuous with the outer dense components of 

 the two apposed "unit" membranes (Fig. Sb, arrow). A diagram- 

 matic representation of this type of desmosome is shown in Figure 

 3c. The two plasma membranes are joined by septa which may 

 possibly have direct connections to the outer components of the 

 apposed plasma membranes. Lack of continuity, as illustrated at 

 B, is more commonly seen than continuity shown at A, so it is 

 uncertain which configuration is more accurate. Perhaps both 

 conditions occur along the course of the same septum. 



Another type of intercellular attachment occurs in hydra where 

 muscle processes are apposed end to end. Myofilaments appear to 

 insert into regions of increased density and the two cell surfaces 

 are maintained always in close approximation. This relationship, 

 shown in Figure 4 (a, h) resembles the intercalated disc of verte- 

 brate cardiac muscle. The intercalated disc is now recognized as 

 a kind of desmosome (see Sjostrand and Andersson, ref. 10). 

 This type of attachment is particularly clear in longitudinal sections 

 of the epidermis (Fig. 4«). In cross section they appear at the 

 base of the gastrodermis and may be distinguished as irregu- 

 lar, dark streaks in light micrographs (Fig. \h). 



In the basal region of the tentacles, and in the upper part of the 

 column, there is a special type of relationship of the muscle pro- 

 cesses to the mesoglea. This type of attachment may also be identi- 

 fied by light microscopy in favorable preparations. Figure 5a is a 

 light micrograph of a longitudinal section of a tentacle near its 

 junction with the hypostome. Near the mesoglea an area of in- 

 creased density is quite apparent, but the details of its structure 

 are not obvious. A similar region viewed in the electron microscope 

 (Fig. 5i>) shows that the density is caused by a specialized 

 muscle insertion on mesoglea. The attachment is accomplished by 

 means of a narrow finger of epitheliomuscular cell cytoplasm which 

 becomes intimately associated with an area of increased density in 

 the adjacent mesoglea. The cytoplasmic finger contains a condensa- 

 tion of material which appears to be organized into a series of 

 small tubular elements arranged at right angles to the plane of the 

 plasma membrane (Fig. 6). The disposition of these tubules sug- 



