632 ANATOMY OE VERTEBRATES. 



arteries diverge from a common trunk, and the venous channels 

 become more concentrated towards the heart. A venous sinus is 

 formed behind the auricle, and this is divided by a valvular struc- 

 ture from the ventricle, which now is larger than the bulbus. 



The changes of the primitive vascular arches into the arterial 

 trunks arising from the adult heart are effected more speedily and 

 directly in allantoic Reptiles than in Batrachia, pp. 519, 520, 

 because no branchial organs and vessels are developed : such spe- 

 cial respiratory apparatus for a temporary arpnatic existence is 

 interposed in the anallantoic species, and interrupts, so to speak, 

 the course of the transformation which is now to be described. 



The primitive distribution of blood from the ' bulbus ' of the 

 embryonal heart in ' Vertebrates ' is by a series of symmetrical 

 arches on each side the alimentary canal, dorsad of which those 

 loops or arches unite to form or join the aortic trunk : they relate 

 to the primitive segmental character of the embryo, co-existing 

 with maxillary, mandibular, hyoidean, and scapular segments, all 

 of which at this period are unclosed arches on the sternal aspect 

 of the fore-part of the body. 



The four or five primitive vascular arches have no essential 

 relation to gills, any more than the clefts or depressions between 

 the budding piers of the maxillary, fig. 444, a, mandibular, ib. b, 

 and hyoidean, ib. c, arches are necessarily the precursors of the 

 branchial openings. Both primary structures exist in the embryo 

 of those vertebrate classes that never possess the true branchial 

 organs : these are superadded developements upon the common 

 segmental type of pleurapophysial and pleurarterial parts, which 

 developements are peculiar to Fishes and Batrachians, persisting 

 in the first, and vanishing in most of the latter Vertebrates. 

 Of the three vascular arches on each side by Avhich the blood 

 passed from the bulbus to the dorsal vessel, the hindmost are 

 progressively converted, with the growth of the lungs, into the 

 ' pulmonary arteries,' each retaining a connection with the second 

 pair of vascular arches; the third, or anterior pair, with the 

 developement of the head and fore-limbs, in like manner become 

 diverted to their exclusive service, but for a time retain a con- 

 transparent globular cells, with a large granulated nucleus (mesoblast, Ag.), attached 

 to the wall. ' By the application of water the nucleus bursts and the whole granular 

 contents come out, but still retain their globular state and appear to have a membrane 

 about them. From this it would appear that the apparently granular contents of the 

 mesoblast constitute, in reality, an entoblast (nucleolus), which fills the mesoblast.' 

 The flat elliptic form is not attained until very late. The mesoblast is faint and 



homogeneous to within a short time before extrication of the turtle: in the adult it 

 contains a darker entoblast. ccc. p. 617. 



