58 ZOOLOGY. 



found in the marrow-cavities of certain bones, particularly 

 the ribs. The yellow marrow of other bones may also 

 possibly give rise to red corpuscles, but this tissue consists 

 very largely of fat (see below). 



The white corpuscles, in spite of their power of growth 

 and division, must also die off, for new ones are constantly 

 being added to the blood, without causing any net increase 

 in their numbers. These white corpuscles are mainly 

 developed by division of the cells of those "lymphatic 

 glands " already mentioned as occurring here and there on 

 the course of the lymphatics. These structures are not 

 glands at all, in the strict sense of the term : they are 

 masses of lymphoid or adenoid tissue, which resembles con- 

 nective tissue, but has a much more fluid matrix with 

 delicate fibres forming a network. In the interstices of the 

 network are abundant actively-dividing corpuscles. Lym- 

 phoid tissue is the chief constituent of Peyer's patches, and 

 the walls of the sacculus rotundus, and appendix vermiformis. 



14. Adipose Tissue. In all the tissues mentioned so 

 far the cell-products are ectoplastic. As a common example 

 of endoplastic products we may mention fat. Under certain 

 conditions the corpuscles of connective-tissue secrete fat, 

 which appears as globules in the protoplasm (fig. 18). As 



Fig. 18. DEVELOPMENT OF A FAT-CELL. 



the amount secreted increases, these globules unite into 

 larger and larger drops, until finally we have a thin bladder, 

 as it were, of protoplasm bulged out by a huge fat drop. 

 Connective tissue, in which most of the cells are thus 

 fat-laden, is called adipose tissue. This is abundant under 

 the skin of a well-fed rabbit, and also around the kidneys. 



15. Notochordal Tissue. Another and more striking 

 example is afforded by notochordal tissue a tissue practically 



