l-i 



TUB AMERICAN MUSEUM JOUEXAL 



become intense. By it cells that at first 

 could not be distinguished from one an- 

 other eventually develop into cornea, 

 iris and pupil, lens, and retina— the 

 retina alone having ten different layers ! 

 — and the strong and tough outer pro- 

 tecting fibrous envelope of the eye ; the 

 three bony semicircular canals of the 

 internal ear lined with their soft mem- 

 brane, the wonderful spiral cochlea in 

 which are distributed the nerve fila- 

 ments forming the ultimate organ of 

 hearing; the three little bones of the 

 ear attached to one another and by 

 flexible membranes to the internal ear, 

 and externally also to the drum of the 

 ear. 



And as if all this were not enough, 

 there are certain other cells which be- 

 come segregated — shall we say segre- 

 gate themselves? — from the others and 

 develop on an entirely different plan 

 from any of the others. Some of them 

 form a long hollow tube, the esophagus, 

 followed by a dilated part, the stomach, 

 and then the intestines. The stomach 

 has a wholly different function from 

 all the rest of this long tube, which 

 is about twenty-five feet long in man 

 and far longer in some of the lower 

 animals. The stomach portion of the 

 tube suddenly narrows again at the py- 

 lorus, the outlet from the stomach into 

 the small intestine, and finally widens 

 anew into the large intestine. Through- 

 out its length this digestive tube is 

 lined with different sorts of glands to 

 aid the different stages of digestion. 

 Connected with this long digestive tube 

 are two large accessory solid organs — 

 the liver to secrete bile, the pancreas 

 (the "sweetbread" in animals) to fur- 

 nish a juice to aid digestion. 



At the same time other cells are 

 segregated at one point to form the 

 spleen. Its function is still, in part, as 

 yet obscure. Its presence is not essen- 

 tial to life for many times it has been 

 removed, and the patients get along 

 without it, as they do also without any 

 gall bladder or appendix. 



Still other cells are segregated into 

 two masses of cells which form the two 

 kidneys. The character and function 

 of these cells become wholly changed. 

 Their secretion is wholly different from 

 that of the glands in connection with 

 the mouth (for saliva), in the stomach 

 (for gastric juice), in the liver (for 

 bile), in the pancreas (for pancreatic 

 juice), in the intestine (for the intes- 

 tinal juice) all aiding in digestion, that 

 is, in the nourishment of the body. The 

 cells of the kidney select from the blood 

 such of the waste products as otherwise 

 would accumulate in the blood and 

 quickly threaten life. Hence their 

 enormous importance. As a surgeon I 

 would far rather operate on a patient 

 with very grave heart disease than on 

 one whose kidneys were seriously out of 

 order. 



The sweat glands of the skin as I 

 have already pointed out are analogous 

 to the kidneys, and supplement, or may 

 even replace them. 



All the stages of development of all 

 these different tissues have been dili- 

 gently worked out, chiefly in the ferti- 

 lized egg of the hen. One can abstract 

 an egg from an incubator at any defi- 

 nite time, one hour, two, three, four 

 hours, or on the second, third, fourth, 

 fifth day and so on, carefully open the 

 egg shell, harden the embryo chick in 

 certain fluids and then cut it into very 

 thin slices which are kept in their serial 

 order. These "serial sections" are so 

 thin that the light easily passes through 

 them. This enables us to study each 

 one in detail. They can be cut also 

 lengthwise, crosswise, or obliquely. By 

 these means the embryologist can ex- 

 amine each section, each system, each 

 organ, or even a part of any organ. By 

 suitable stains one can differentiate 

 and study the individual cells of the 

 various tissues. 



Occasionally accident will furnish a 

 htiman embryo in which similar exami- 

 nations by similar methods may be 

 made. These absolutelv establish the 



