SIMPLE TENSILE CONNECTING TISSUES 



61 



carbonate. This grows until it gets too large for the cell and its end 

 sticks out. 



At this time it is used for a rigid support for the body of the sponge in 

 connection with many other similar rods. The 

 spicules (Fig. 63, A, B\ as these structures are 

 called, assume a great variety of forms. (For 

 the supporting cells in certain integuments, 

 see Chapter XX on the "Integument.") 



Technic. In general, fixatives containing 

 acids, particularly acetic acid, should be 

 avoided in the preparation of connective tis- 

 sues. The rigid forms are least damaged, 

 especially when they are chitinous in nature. 

 Flemming's fluid and sublimate should both 

 be used for such examples as are used in this 

 part. Sublimate is neutral in its action, and 

 is very useful where it is desirable to study 

 the cell structure as well as a spicule or other extra-cellular formation. 

 This sublimate must always be carefully removed by means of iodide 

 of potassium (see chapter on technic). 



FIG. 63. Developing spicules 

 of a sponge. (From SCHNEI- 

 DER after A. MAAS.) 



LITERATURE 



BUTSCHLI, O., 1901. " Einege Beobachtungen iiber Kiesel- und Kalknadeln von 



Spongien," Zeits. f. Wiss. Zool., Band LXIX. 

 BIDDER, G., 1898. "The Skeleton and Classification of Sponges," Proc. of the Royal 



Soc., London. 

 HOLMGREN, N., 1902. " Uber das Verhalten des Chitins und Epithels zu den unterlie- 



genden Gewebsarten bei Insecten," Anat. Anz., Band XX. 

 SCHNEIDER, K. C. " Lehrbuch der Histologie." 



THE SUPPORTING AND CONNECTIVE TISSUES: PRIMITIVE TEN- 

 SILE FORMS 



Examples of cells that bind the parts of a body together are universal. 

 The simplest form is seen to greatest advantage in the embryos of all 

 animals before the extra-cellular substance has assumed such proportions 

 as to obscure the cell and its nucleus. An example is found in the um- 

 bilical cord of a sheep about one quarter advanced in its intra-uterine 

 development (Fig. 64). 



The tissue lying directly under the external epithelium is composed 

 of a number of cells spaced at somewhat regular distances from each other. 

 Each cell has its nucleus placed in the middle of the cell body. This 



