Histogenesis of the Adipose Tissue. 413 



The structure of the tissue will be described from the earliest 

 foetal stages until the fully-formed condition is attained. Some of 

 the fat cells at birth differ only in size from those of the adult 

 animal ; but the final stages of differentiation nearly always take 

 place after birth. 



Material and Methods. The material employed was almost ex- 

 clusively calf foetuses from the 3 cm. stage until about full term. 

 A number of observations have been made on the histological changes 

 occurring in adipose tissue during fattening; but this phase of the 

 subject has not been completely worked out and will only be referred 

 to incidentally. 



A large part of the material was fixed in Zenker's fluid or in 

 Gilson's fluid and embedded in paraffin. The stains used on the 

 sections thus prepared were mainly Mallory's anilin blue and iron- 

 hsematoxylin. Iron-hsematoxylin is especially valuable for this study. 

 When the differentiation in iron-alum is omitted, leaving the hsema- 

 toxylin intense, the finest connective tissue fibers are brought out, 

 though not differentiated from the nuclei and protoplasm. The 

 results obtained by this method are readily interpreted by comparing 

 with a similar section stained with Mallory's anilin blue. Some 

 sections were overstained with iron-hsematoxylin and then moderately 

 differentiated to bring out the Altmann granules and the nuclei. 

 Eosin or Congo red were sometimes used as counterstains after iron- 

 haematoxylin. 



Considerable material was fixed in 20 per cent formalin. Formalin 

 material cannot be stained well with Mallory's anilin blue, but the 

 Altmann granules are brought out well by iron-hsematoxylin after 

 this fixation. Frozen sections were made at practically all stages 

 and stained with Sudan or Scarlet red as controls for the embedded 

 material. The same precautions were taken with Scarlet red and 

 Sudan, as described in Part I. 



A discussion of the older literature of adipose tissue may be found 

 in the papers of Czajewicz and Flemming. The older papers will 

 not be referred to here. It is sufficient to give only the general con- 

 clusions to which the more recent investigations have led. 



As is well known, two theories of the development of adipose tissue 



