EMBRYOLOGY AXD MEDICAL PEOGEESS 17 



embryology he can understand much of what may be expected in this 

 class of variations of structure. 



We will turn now to the fifth and last of our headings, that of 

 morpho-physics. It is only of recent years that methods of experi- 

 mentation, as distinguished from methods of observation only, have 

 been applied to embryological problems. Naturally under the circum- 

 stances many crude experiments have been undertaken, many of 

 doubtful validity; but there have also been many others, soundly 

 planned, rightly executed and fruitful in results. Already the new 

 conclusions constitute an increment both large and precious to our 

 stock of embryological knowledge. One important class of these ex- 

 periments has been based upon the discovery of the possibility of 

 grafting parts of amphibian embryos on to one another; or to get two 

 large pieces of two distinct embryos, or even two halves of two embryos, 

 to grow together. The grafting experiments which have already been 

 made are very numerous. Let me present one or two examples of the 

 sort of results that these experiments yield. If the halves of two 

 species of frog in a very early stage are grafted together, they will 

 unite perfectly, but it is found that the epidermis of the species which 

 forms the anterior half of the graft will spread to a certain extent over 

 the posterior half, thus showing that the skin can actually crawl over 

 the underlying tissues. It is probable, indeed, that the migration of 

 epithelial cells along the surfaces upon which they rest is a very general 

 phenomenon, and plays a very important part in the animal economy. 

 In another series of experiments the embryonic optic vesicle has been 

 removed and grafted on to a new part of the larva. Where the optic 

 vesicle comes in contact with the epidermis it causes the epidermis to 

 form a typical lens for the eye. Thus it is proved that the formation 

 of the lens is not a specific function of that part of the epidermis from 

 which it is normally produced, but is a potential function of the entire 

 embryonic epidermis which may be called forth into activity by con- 

 tact with the growing optic vesicle. I believe that we have in this 

 an illustration of one of the fundamental principles of the establish- 

 ment of structure and that much depends upon the interaction and 

 mutual stimulation of parts. 



Another class of experiments has been conducted by those who 

 have been somewhat jocosely named the ' egg shakers.' An egg during 

 an early stage of segmentation is divided artificially into its natural 

 segments, or into groups of such segments, as the case may be. In 

 many cases this division can be accomplished by shaking the eggs 

 somewhat violently so as to break the segmentation spheres apart; 

 hence the name above quoted. Xow it has been demonstrated that in 

 -'•me cases fragments of a single egg will develop into an embryo per- 

 fect apparently in structure, though only of say half the normal size, 



VOL. LXIX. — 2. 



