ZOOLOGY AND BOTANY. MICROSCOPY, ETC. 541 



were sorted out whose progeny were high-producers, and the others 

 were rejected. The author adheres to his view that the absolute 

 somatic value of a particular hereditary factor or determinant (i.e. its 

 power to cause a quantitatively definite degree of somatic development 

 of a character) cannot be changed by selection on a somatic basis, 

 however long continued. 



Three-legged Kitten.* — W. B. Kirkham and H. W. Haggard 

 describe the structure of a three-legged kitten, the left fore-limb being 

 apparently absent. The scapula showed no coracoid process ; the 

 glenoid cavity was very imperfect and convex rather than concave ; the 

 left humerus was minute and without marrow cavity. It is probable 

 that after starting to grow out normally from the body the limb-bud 

 encountered some obstacle which pressed it against the body and checked 

 its increase in length. Such of it as had already grown underwent 

 further development into bone, muscle, and connective tissue on a 

 reduced scale. 



b. Histology. 



Colloids and Tissue Structure.f — Eaphael Isaacs has experimented 

 in reference to the relation between the properties of colloids and fresh 

 tissue structure, as observed under the Microscope by dissection or 

 compression methods or in hanging drops. The clear-cut pictures of 

 the fixed and stained tissue are absent, as well as the fibrillar structures 

 characteristic of fixed sections. The tissue colloid changes rapidly on 

 removal from the body, swelling or coagulating. 



The experiments made show that when tissues are removed from 

 the body for examination in the living condition, any change in refrac- 

 tion indicates a dehydration, a gelation, or a solution process, which, 

 beyond narrow limits, tends towards irreversible changes in the state of 

 the cell colloids. Both dehydration and gelation increase the refraction. 

 An optical change in the tissue colloids is noticeable only after consider- 

 able change in concentration, but is more readily noticeable if the 

 change is from the sol to the gel state or the reverse. 



The pattern produced on coagulation or fixation is due to a shrinking 

 and to diffusion currents, which cause the granules which are precipitated 

 to whirl around, finally becoming wound up in the parts which coagulate 

 more slowly. The pattern for a dilute and a concentrated colloid is 

 the same, the former yielding a more delicate fixation figure than the 

 latter. The physiological state of a tissue at the time of fixation can 

 thus be worked out. The action of physiological salt solutions in 

 preventing swelling due to acid intoxication is not uniform for all the 

 constituents of the cell. Slight variations in the strength of the salt 

 solution may give false impressions as to the relative densities of the 

 cell constituents, because of the different rates of dehydration. 



* Anat. Eecord, x. (1916) pp. 537-42 (3 figs.). 

 t Anat. Record, x. (1916) pp. 517-22. 



