71 



transitional stage of the heart in the early development of the chick, 

 or the permanent stage in the class of animals just mentioned. 



In order to carry out the analogy, and to endeavour to exhibit the 

 successive stages of development of this important organ in the 

 human embryo, I have inserted, in the illustrations which accompany 

 this paper, two drawings, made also from dissections of my own, 

 which represent the heart of the embryo at two periods a little in 

 advance of the present case ; the one (fig. V.) at the beginning, and 

 the other (fig. VI.) at the end, of the second month, as far as the age 

 could be estimated from the size and development of the foetus. 



The first figure (V.) represents the heart as composed now of two 

 auricles (a and b) instead of one, but still having only one ventricle 

 (c), thus symbolizing the heart of the class of vertebrate animals next 

 in advance of fishes, namely reptiles, in which the organ is thus 

 constituted. 



Again, the second figure (VI), representing the heart of a foetus at 

 the end of two months, shows a still further advance, for now 

 the ventricle (c) has become divided into two chambers, thus as- 

 suming the permanent character. The thin membranous, leaf-like 

 auricles (a and b), however, still exhibit traces of a primitive stage of 

 development, which character may also be traced in the yet un obli- 

 terated bulbus arteriosus. 



In conclusion, I wish to be understood as having offered these and 

 other observations contained in this brief account, as entirely open 

 to revision and correction, should further investigations prove such 

 to be required. For the reasons already specified, the opportunities 

 of procuring perfect specimens for examination are so rare, that 

 it is difficult to obtain corroborative evidence by contrasting speci- 

 mens one with another in precisely the same stage of development. 

 Hence, many observations so made, must, for a time at least, rest 

 upon the evidence which a single specimen can afford : for neither is 

 much help to be derived from many of the published descriptions 

 (the authors often not agreeing in their accounts), nor are all the de- 

 lineations of early ova which have been put forth free from the sus- 

 picion of some degree of imagination, at least, having been exercised 

 in aid of these points, which imperfect instruments, or injured and 

 imperfectly developed specimens, may have left obscure. 



The only correction for these impediments to our advance in a 

 knowledge of human embryology, is for every one who has the op- 

 portunity of making careful examination of perfect specimens, to 



