THE TISSUES. 



79 



is colored, while the granula remain colorless. The amount of chromic 

 acid used (0.25 to i%) varies according to the object treated ; if molyb- 

 date of ammonium alone be used, the nuclei will appear homogeneous, 

 while if an excess of chromic acid be employed, the nuclei will appear 

 coarsely reticulated. This method leads to the formation of granula in 

 the cells as well as in the nucleus. 



Butschli's Foam -structure. Fixing is done either in picric acid 

 solution or in weakly iodized alcohol. The specimens are then stained 

 with iron->hematoxylin /. e. , first treated with acetate of iron, rinsed in 

 water, and transferred to a 0.5% aqueous solution of hematoxylin (simi- 

 lar to the method of R. Heidenhain). Very thin sections are required 

 (y 2 to i IL}. Mounting is done, when the lighting is good, in media 

 having low refractive indices, which emphasize the alveolar or foam -like 

 structure of the protoplasm. Of various animal objects, Biitschli espe- 

 cially recommends young ovarian eggs of teleosts, and blood -cells and 

 intestinal epithelium of the frog, etc. It is still a matter of uncertainty 

 whether or not the structures are actually present in living protoplasm. 



II. THE TISSUES. 



The first few generations of cells which result from the segmen- 

 tation of the fertilized ovum have no pronounced characteristics. 

 They are embryonic cells of rounded form, and are known as blas- 

 tomcres. As they increase in number they become smaller and of 

 polygonal shape, owing to the pressure to which they are subjected. 

 From the mass of blastomeres, known as the morula mass, there 

 are formed, under various processes described under the name of 

 gastrulation, two layers of cells, the so-called primary germ layers, 

 of which the outer is the ectoderm, the inner the entoderm. To the 

 primary germ layers is added still a third layer, called the mcso- 

 derm ; it is derived from both the ectoderm and entoderm, but 

 principally from the latter. From these three layers of cells, known 

 as the primary blastodermic layers, are developed all the tissues, each 

 layer developing into certain tissues that are distinct for this layer. 

 In their further development and differentiation the cells of the blas- 

 todermic layers undergo a change in shape and structure character- 

 istic for each tissue, and there is developed an intercelhdar substance 

 varying greatly in amount and character in the several tissues. In 

 the tissues developed from the ectoderm and entoderm the cellular 

 elements give character to the tissue, while the intercellular sub- 

 stance is present in small quantity ; in the majority of the tissues 

 developed from the mesoderm, the intercellular substance is abun- 

 dant, while the cellular elements form a less conspicuous portion. 



The tissues derived from the ectoderm are : 



The epijdermis of the skin, with the epidermal appendages and 

 glands ; the epithelium lining the mouth, with the salivary glands 

 and the enamel of the teeth ; the epithelium and glands of the nasal 

 tract and the cavities opening into it ; the lens of the eye and retina, 



