The Develoiomont of the Heart in Shad. 225 



The portion moyenne appears as a cord of mesodermal cells on 

 either side, intervening between the somital and lateral mesoderm, 

 which tends to remain in contact with the lateral plate. It extends 

 from the middle of the otocyst forward to the anterior end of the 

 lateral plate (see Fig. 4A). Small posteriorly, the portion moyenne 

 becomes larger anterior to the mandibular pouch; here and there 

 it comes into immediate contact with the somital mesoderm. Where 

 contact occurs at this stage, the distinction between portion moyenne 

 and somital portion of the head mesoderm cannot be made out because 

 there is, as yet, no differentiation between the cells belonging to these 

 two parts of the mesoderm. In spite of the fact that in occasional 

 sections the portion moyenne is not quite clearly defined, it forms, 

 as a whole, a perfectly definite structure. 



That the portion moyenne, as found in shad, is directly comparable 

 to that described with such admirable distinctness in trout by Swaen 

 and Brachet cannot, I think, be doubted; for this reason I have 

 ventured to adopt the term employed by them rather than run the 

 risk of confusion by the unnecessary introduction of another name. 



It has been said that the endocardium is derived from the portion 

 moyenne of the mesoderm ; before proceeding to trace the formation 

 of the endocardium it may be stated that special care has been taken 

 to determine whether the entire longitudinal extent of the portion 

 moyenne is involved in the production of endocardium. In order 

 to settle this question (among others) a series of four wax-plate 

 reconstructions has been made from stages during which the cells 

 of the portion moyenne are undergoing migration and differentia- 

 tion to form the endocardium. The result indicates that the endo- 

 cardium, together with the central aorta, is derived exclusively from 

 that part of the portion moyenne originally situated anterior to the 

 transverse plane passing through a point midway between the man- 

 dibidar and hyoid entodermal pouches. That the posterior part of the 

 portion moyenne takes absolutely no share in the formation of the 

 endocardium is an important point which will be referred to later. 



Fig. 4 is a diagrammatic representation of the ventral surface 

 of a wax-plate reconstruction of the pericardial region of a shad's 

 head. Stage of 18 somites (the embryo is one hour and a half older 



