190 PROFESSOR MARSHALL AND EDWARD J. BLES. 



the branchial arches, immediately behind the corresponding primary 

 aortic arches. From the secondary arches outgrowths arise which 

 open into the primary arches, and which form the capillary loops of 

 the gills, both external and internal. The primary arches lose their 

 connection with the truncus arteriosus, and the course of the blood 

 in each branchial arch is now from the heart to the secondary arch 

 or afferent branchial vessel, then through the capillary loops of the 

 gill into the primary arch or efferent branchial vessel, and so on to 

 the dorsal aorta. 



At the time of the metamorphosis the primary arches re-acquire 

 their connection with the truncus arteriosus, and so afford a direct 

 passage from the heart to the dorsal aorta, by following which 

 the blood can escape the gill capillaries altogether. The gills and 

 the secondary arches atrophy, while the primary arches remain as 

 the permanent aortic arches of the adult. 



Maurer agrees with Boas in deriving both the pulmonary and 

 the cutaneous arteries from the fourth arch, the third arch atrophying 

 at the time of the metamorphosis. 



Our own results agree very closely with Maurer's, which they 

 confirm in most of the essential points. Some differences occur, 

 which will be noticed in due course. 



I. THE EARLY DEVELOPMENT OF THE HEART. 



The heart in its early stages of development lies very far forward 

 on the under surface of the head, below the floor of the pharynx, 

 above and slightly behind the sucker, and immediately in front of 

 the commencing liver. 



Its position and relations in an embryo of 3| mm. length are 

 shown in sagittal section in Plate XIIL, Fig. 2, and in transverse 

 section at a slightly later stage in Fig. 3. 



In Fig. 3 the mesoblast at the sides of the pharynx is unsplit, but 

 in the floor of the pharynx, on the ventral surface of the embryo, it 

 is divided into somatic and splanchnic layers, M O and M P, 

 separated from each other by a considerable space, C A, which 

 will become later on the pericardial cavity. Of the two layers of 

 mesoblast, the outer or somatic layer keeps close to the epiblast ; 

 the inner or splanchnic layer is in close relation with the hypoblast 



