396 PRACTICAL HISTOLOGY. [XXXVIII 



villus, so that only a small amount of connective tissue intervenes 

 between the vessels. Each villus is covered on its surface by a 

 layer of epithelium, which, however, is thin at one part and thick at 

 another. Especially at the ends of the villi are large granular masses 

 of protoplasm containing many nuclei, but one cannot make out a 

 separation of these masses into cells. They often contain vacuoles. 

 The arangement of the blood-vessels may be followed from the 

 distribution of the blood-corpuscles (fig. 368). 



Small portions of a placenta are also to be hardened in Miiller's 

 fluid and stained in bulk in borax-carmine. Individual villi may 

 be isolated in dilute alcohol or osmic acid. 



6. Injected Placenta. Examine a vertical section of a placenta 

 with the fo3tal blood-vessels injected, say blue, and the maternal 

 vessels red. Observe how the one set interlocks with the other, yet 

 both systems are closed and do not communicate with each other. 



LESSON XXXVIIL 



TO MAKE PREPARATIONS RAPIDLY PROM 

 FRESH TISSUES. 



IT is of the utmost importance that the student should be acquainted 

 with the methods of making preparations from fresh tissues placed 

 in his hands. The following is an outline of the work that each 

 one can readily do for himself if supplied with a pithed frog, or 

 other suitable material. 



A. From a Frog. 



1. Corneal Corpuscles. With a sharp pair of scissors cut out 

 the cornea. Divide it into two parts. 



(a.) Treat one by the lemon-juice method (p. 79). 



(/>.) Treat the other part by placing it direct into .5 per cent. 



AuCl 3 (half an hour) ; wash in distilled water ; place in 



a saturated solution of tartaric acid at 50 C. until the 



gold becomes reduced (p. 79). 

 ('.) A cornea may be placed fresh in dilute methylene-blue 



(i : 300 normal saline). Mount in picrate of ammonia 



glycerine (p. 192). 



2. Corneal Lymph-Spaces. Remove the eyelids, expose the 

 surface of the other cornea, scrape olf the epithelium, and rub it 



