BY JAS. P. HILL AND C. J. MARTIN. 71 



Fig. 4, — Transverse section passing through about the middle region of the 

 head plate of mesoderm {hp. mes.j. The amnio-cardial vesicles 

 [amc. c. V.) form its outer limit. ( x 70.) 



Fig. 5. — Transverse section between the posterior limit of the head plate of 

 mesoderm and the auditory plate. The section passes through 

 the anterior end of the lateral heart Anlage — the endothelium 

 [ht. end.) of which here consists of two tubes. Mesially to the 

 hear-t Anlage the pharyngeal entoderm [ent. ph.) is visible. 

 Mesially to the latter the mesoderm is interrupted over a small 

 area, (x 70.) 



Fig. 6. — Transverse section through the middle of the lateral heart Anlage. 

 The endothelium here forms a single tube, though traces of a 

 septum are still visible in it. The (splanchnic) mesodermal wall 

 {spl.) of the heart has here a semitubular shape. The section 

 also passes through the 3rd neuromere of the hind-brain {n'".) 

 apparent as a thickening of the medullary plate and through the 

 grooveil auditory plate [and.). ( x 70.) 



Fig. 7.— Transverse section through the region immediately in front of the 

 1st somite. The paraxial mesoderm {mes. ax.) here exists in the 

 torm of an arched plate, below which are numerous stellate cells. 

 The lateral mesoderm is not completely split, the coelom being 

 represented by several interrupted spaces. The median sulcus 

 (Riickenfurche) [d. fr.) of the medullary plate is here very 

 marked and the notochord is now quite free from the entoderm. 

 (X 70.) 



Plate XI. 



Fig. 8. — Transverse section through the 6th somite. The somite somewhat 

 oblong in section, is seen to extend mesially below the medullary 

 plate and to possess a very distinct cavity [m. s. c). The somite 

 is separated from the lateral mesoderm by the Anlage of the 

 Wolffian body {w. h. ) Ventrally the latter is distinctly connected 

 with the lateral mesodenn, while the connection with the somite 

 is not so distinct. The lateral mesoderm splits sonie distance out 

 into a thick somatic layer {-^om:) and a thin splanchnic layer 

 (•s;;^.). The longitudinal strand of cells [le.) immediately 

 external to the edge of the medullary plate and passing inwards 

 below it is also visible. ( x 70.) 



Fig. 9. — Transverse section through the 7th somite. In this section the 

 Anlage of the Wolffian duct (w;. d.) is seen to be distinct from 

 the Anlage of the tubule [iv. t.) underlying it. The latter is 



