THE MICROSCOPE IN ANIMAL HISTOLOGY. 201 



brane, through which the plasma transudes, and cells of 

 glandular epithelium. A vascular network exists on the 

 surface of the membrane, from which the material of the 

 secretion is obtained. This membrane may be a simple 

 follicle, or tube, as in the mucous membrane, or system of 

 tubes, as in the kidneys, a convoluted tube, a simple open 

 vesicle, a racemose aggregation of vesicles, or a close cap- 

 sule which discharges itself by bursting. (Plate XXI, 

 Fig. 158). 



4. Vascular Tissue. The smallest bloodvessels and lym- 

 phatics, called capillaries, are minute tubes, consisting of a 

 series of flattened epithelial cells, and containing stomata, 

 or openings through which white or red blood-corpuscles 

 may occasionally pass (Plate XX, Fig. 159, a, 6). The 

 larger trunks have, in addition to the cellular layer, one 

 of longitudinally striated connective tissue, a middle coat 

 containing transverse muscular fibres, and an external coat 

 of connective tissue (Plate XXI, Fig. 159). The distribu- 

 tion of the capillary bloodvessels is various, according to 

 the nature or function of the organ or tissue in which they 

 are found. 



DEVELOPMENT OF THE TISSUES. 



It has been stated, page 125, that reproduction in the 

 higher animals consists of an ovum fecundated by contact 

 with a sperm-cell, or spermatozoid. The ovum consists 

 of a germinal vesicle, containing one or more germinal spots, 

 and included within a vitellus (a yelk) which is surrounded 

 by a vitelline membrane, which may have additional invest- 

 ments in the form of layers of albumen and of an outer 

 coriaceous or calcified shell. 



The first step in the development of the embryo is the 

 division of the vitelline substance into cleavage-masses, at 

 first two, then four, then eight, etc. This process of yelk- 

 division may affect the whole yelk or a part of it, and re- 

 sults in the formation of a blastoderm, or embryogenie 



