The Development of the Heart in Shad. 223 



shown in other teleosts by Oellacher, '73, H. E. Ziegler, '87, Swaen 

 and Brachet, '00, and by Sobotta, '02. Nothing has been found in shad 

 supporting the view that the endocardium arises from entoderm alone 

 or from entoderm and mesoderm together. 



The particular part of the mesoderm from which the endocardium 

 is derived is a, bilaterally symmetrical, cord of cells on either side 

 immediately adjacent to the medial borders of the lateral plates; 

 of this Swaen and Brachet have given a careful description (as 

 found in Trutta fario) and to it they have applied the term "Portion 

 moyenne du mesoblaste." In shad at a stage of 15 somites the 

 portion moyenne is recognizable throughout most of its, eventual, 

 longitudinal extent; although comparison with earlier stages shows 

 that it has been recognizable for some time but in a less advanced 

 stage of development. 



In order to gain a clear conception of the portion moyenne, as 

 it occurs in shad at the stage of 15 somites, it is necessary to enquire 

 into the cause of its distinctness from the remainder of the head 

 mesoderm. There are, as far as I can see, no features in the cells 

 composing the portion moyenne which distinguish them at this stage 

 from other mesodermal cells. The portion moyenne, as a whole, is 

 distinguishable from the lateral plate by reason of the orientation 

 of the cells of the latter (where this has occurred) around the, now 

 virtual, coelon; see Figs.^^ 35^ 3C, 3E and 3F. The lateral plate, 



"To facilitate comparison between the sections sliown in figures 3, 4, 5, 6, 7, 

 10, 11, and 12 the same letter lias been used througliout to indicate a certain 

 region. Seven regions have been selected as follows : 



A is near the, eventual, anterior limit of the lateral plates. 



B is between A and the mandibular entodermal pouch. 



C is through the mandibular entodermal pouch. 



D is midway between C and the gill anlage spoken of in the text as the 

 hyo-branchial. 



E is a short distance in front of the hyo-branchial anlage. 



F is through the hyo-branchial anlage and the anterior part of the develop- 

 ing otocyst. 



G is some distance posterior to the branchial region of the pharynx. 



In figures of sections P.M. indicates portion moyenne. 



The anterior aspect of the section is always represented ; the structures on 

 the right side of the embrj^o will, therefore, appear on the left side of the figure, 

 and vice versa. 



