26 



T. H. BURLEND, 



tional error, since I found it difficult to decide in some sections 

 whether the duct was free from the underlying mesoderm or not. 



NB. X I V3 means one-third of segment X. 



XI 1 2/3 means two-thirds of segment XI etc. 



A further source of error in the above table may have arisen 

 from a miscalculation of the number of protovertebrae on each side, 

 since I have estimated the age from the number of protovertebrae. 

 This error could not be more than one protovertebra in any case and 

 so would not affect the main results which I have obtained. 



Leaving these very early embryos we pass on to the study of 

 slightly more advanced stages. 



Embryo I. 



Stained for 48 hours in Geenacher's Boraxcarmine. After clearing 

 in xylol I counted 18 somites. The somite formation has almost 

 extended to the hind end of the body and the heart is now pro- 

 minent. 



Mandibular pouch nearly open 



Hyoid pouch developing 



Sectioned, and mounted the sections on 7 slides (45 on each 

 slide). 



