Frederick Adams Woods 317 



tlie gut migrates as a whole over the surface of the mesenchyma or that 

 the gilt, as a whole, sinks further and growth of tissue takes place in 

 its lower parts. 



However, there comes a stage now which seems to me incompatible 

 with any view except that of independent motion through the tissue. 

 This is the transition between Fig. 13, 15 mm., and Fig. 11, 19 mm. 



In the 15 mm. embryo there were 276 primitive ova in the posterior 

 epithelium of the ccelom, 29 in the mesentery and 11 ventral to this 

 point. Of these 11 occupying ventral positions there were 28 which are 

 especially interesting in trying to prove this point since these lie around 

 the intestine and even directly under it. Xineteen of these 28 lie in 

 the territory between a and b and 9 lie between b and c. 



In the 19 mm. embryo there were 212 primitive ova in the genital 

 region, 19 in the mesentery and 11 ventral to this point. Nine of 

 these were in the region b-c. None were in a-b, though the entire 

 embryo was searched. The other 2 lay much further away, near the 

 remains of the yolk. This was confirmed by a study of an 18 mm. 

 embryo. 



These changes between the 15 and 19 mm. are entirely compatible 

 Avith migration through the tissues and it seems to me with no other 

 view. It is not conceivable or at least extremely improbable that any 

 special growth should raise cells from point a to point c. The only 

 other alternative would be degeneration in situ of the cells between 

 a-b in the 15 mm. embryo. 



I have looked for such degeneration in the cells under the alimentary 

 tract, but have never observed any such evidences. Whenever they 

 were found they have always, with very few exceptions, shown out clearly 

 and with no transitions wherever their positions may have been. So it 

 seems that the changes in the early stages are due mostly to changes 

 in relative growths of the ditferent parts of the embryo and that in the 

 later stages at any rate we must add an independent migration through 

 the epithelial cells. This independent migration, though remarkable 

 and difficult to explain, is somewhat paralleled by young nerve cells 

 whose movements through tissue are acknowledged to take place. It 

 is very hard to understand how these cells can migrate even for a little 

 distance through the cells of a columnar or low cuboidal epithelium 

 and practically never fall out or wander into the underlying tissue. Yet 

 such appears to be the case, especially in the stages between 11 and 

 19 mm. 



It may be that some chemical forces, which we at present in our ig- 

 norance call chemotaxis, are factors taking an important joart in the 

 development of tissues. 



