THORACIC DUCT DEVELOPMENT IN THE PIG 403 



fixed in picro-sublimate and chrom-aceto-formaldehyde, and 

 several others were stained with borax carmine in toto and coun- 

 terstained with picric acid or blue de Lyon. Picro-sublimate is 

 very unsatisfactory in its preservation of tissue and should not be 

 used. For excellency of fixation and differentiation of vascular 

 structures the first method mentioned proved by far the best. 

 Not only did the sections show a beautiful transparency of color 

 and a strong contrast when this method was carefully followed in 

 detail, but they also produced the most favorable microphoto- 

 graphs. 



A few embryos were injected with India ink by the writer. 

 Series 23a was prepared and injected by Professor Sabin and 

 serves as an excellent critical stage in the development of the 

 thoracic duct. 



Table 1 which represents only a partial catalogue of the Prince- 

 ton Collection of pig embryos enumerates the series studied and 

 reconstructed. They are arranged and grouped according to the 

 developmental status of the thoracic duct region in each, and 

 therefore certain embryos precede others of slightly lesser length. 

 On the whole, however, this system of gradation corresponds very 

 closely to gradations by length, the little inconsistencies here 

 and there being of trivial account when we consider that measure- 

 ment can be at most of only approximate accuracy, and that 

 fluctuations in growth are not at all infrequent. 



The length of each embryo, excepting Sabin's series 23a, was 

 obtained after fixation, when the danger of possible mutilation to 

 the embryo is least, and represents the crown-rump measurement, 

 viz., the distance between the crown of the head and the base of 

 the tail. Sabin's series being measured before fixation is con- 

 sequently longer; but when it is considered that the processes of 

 fixation, hardening and dehydration reduce the absolute length 

 of such an embryo by 1 or 2 mm., this discrepancy in size dis- 

 appears and it is seen to fit in smoothly with related series. 



All of the embryos were sectioned transversely to 15 and 20 

 micia in thickness. The significant sections of the series selected 

 for reconstruction were carefully drawn with the aid of the Edinger 

 drawing apparatus, the details then confirmed by the high power 



