xxn PREFACE. 



the phenomenon of the appearance of the lattei', in which the 

 blastema! is accompanied by a vascular arcli, ■\'\'ith clefts inter- 

 vening between contiguous arches, especially at the fore part of 

 the embryo, has led to the idea that a reptile, bird, or mammal, 

 is a fish before it becomes what it is tending to. True it is, that 

 the embryos of these air-breathers float in fluid, and not any of 

 them breathe the air until birth or exclusion, or near to exclusion ; 

 but they do not breathe water : the oviparous air-breather has one 

 kind of temporary lung, the mammiferous embryo another kind, 

 each alike special to the class. From the vascular loops accom- 

 panying the hffimal arteries branchias are not developed ; one only 

 of the interhajmal fissures is deepened on each side, brought into 

 communication with the i}harynx, and straightway converted into 

 the ' eustachian tube,' according to the precocious rate of gro\vth 

 and development characteristic of the special organs of sense 

 and their appendages. No true branchial or piscine breathing 

 apparatus is at any time, or in any degree, manifested in the 

 embiyo of an air-breathing vertebrate. The deepening and open- 

 ing of several interhfemal fissures in the embryo of a perch, and 

 the subsequent course of development therewith of gill-arches and 

 "■ills, Avith their subservient mechanism of branchiostesal ravs and 

 the opercular lid or door, are as distinctive manifestations of the 

 original nature of the fish, as is the vascular lining of the egg that 

 of the bird, or the vascular arrangement for borrowing breath 

 from the mother that of the foetal mammal. 



At the incipient stages of these provisional and deciduous 

 respiratory conditions the circulation in the embryo lizard, fowl, 

 beast, is like that of a fish in its simplicity ; but, as Teevieaxus' 

 rightly remarked, it is far from being identical ; there are, indeed, 

 characters of the circulating organs at this grade of simplicity, 

 which not only distinguish the embryo of the air-breather from 

 that of the water-breather, but also the embryo of the mammal 

 from that of the bird or reptile ; so soon is the course of deve- 

 lopment afl:ected by the specific taint ! 



1 G. E. in 'Zeitschrift furPliysiologir,' vol. iy. ; ;iiul • Edinluirgii Ni-^v riiilosophical 

 Journal,' 1832, vol. xiii. p. 75-86. 



