vi OEIGIN OF THE HEAET AND VESSELS 367 



corpuscles become differentiated with further development into 

 specialized strains — Erythrocytes or Eed corpuscles and the various 

 'types of Leucocytes ; for an account of this in Lepidosiren the student 

 may be referred to the beautiful memoir by Bryce (1905). 



The mode of origin of the vascular system in the holoblastic 

 vertebrates in general seems to resemble in its main lines that 

 described above. The chief question that has given rise to dis- 

 cussion is one which in the opinion of the present writer resolves 

 itself very much into a question of mere verbal expression — namely 

 whether it is more correct to state that the first rudiments of the 

 vascular system (or parts of them) are derived from mesoderm or 

 from endoderm. When it is borne in mind that the mesoderm of the 

 Vertebrate is essentially a derivative of the endoderm, it will be 

 realized that the question is of minor importance whether or not 

 special portions, such as rudiments of the vascular system in particular 

 cases, lag behind the main mesoderm in their separation from the 

 endoderm, so as to originate from the latter directly instead of from 

 the already differentiated mesoderm. 



Origin of the Heart in Meroblastic Vertebrates.— The 

 heart in its earliest stages shows in meroblastic vertebrates generally 

 a set of conditions quite similar to those met with in holoblastic 

 forms. In Elasmobranchs (Fig. 176, A) the first obvious rudiment 

 of the heart is in the form of a number of cells of irregular shape 

 which make their appearance on each side of the foregut, between it 

 and the splanchnic mesoderm, from which latter they are apparently 

 derived. As the foregut separates off from the endoderm of the yolk 

 the irregular row of these cells shifts downwards and towards the 

 mesial plane, so as to form a single elongated group underlying the 

 foregut 1 (Fig. 176, B). The individual cells unite together and form 

 a syncytial mass containing vacuolar spaces (Fig. 176, 0, enc). Finally 

 (Fig. 176, D) this elongated mass assumes a tubular form, its vacuoles 

 coalescing and increasing in volume to form a wide cavity, while the 

 protoplasm becomes thinner and forms the endothelial wall. The 

 myocardium (mc) comes to surround the endothelial tube in exactly 

 the way described for holoblastic forms. 



In the Sauropsida again the phenomena are similar. To take as 

 an example the Fowl : about the stage when two or three segments 

 are formed isolated cardiac cells begin to appear on each side between 

 the endoderm and splanchnic mesoderm. These cells increase in 

 number and form a longitudinal tract on each side, the two con- 

 verging and meeting anteriorly. As the foregut becomes constricted 

 off the two cardiac strands come together from before backwards in 

 front of the yolk - stalk though even at the eight - segment stage 

 they are not completely fused. The endothelial rudiment now 

 forms a loose syncytial spongework with large meshes containing 



1 Riickert (1888) believes them to be reinforced by cells derived from the ventral 

 endoderm of the foregut. 



