114 FOURTH REPORT — 1834. 



embryo advances, we observe some organs superadded, though 

 still in a very simple form ; so that here also a certain resem- 

 blance subsists between the embryo in this second stage and 

 animals a little more complex. As we continue to observe 

 the embryo of the higher family, we see organs come into 

 view, some of which are meant only for a transitory purpose 

 and disappear ; some which have no pvu-pose during fcetal 

 life, but ai'e meant for an ulterior use. Here the resemblance 

 between the eml)ryo of the higher form, and the animal of the 

 lower form with which we may most favoiu'ably compare it, is 

 found to be less close. We find that the animal has organs suited 

 to the activities with which it is endowed, which are not to be 

 found in the embryo. Even if the two exist under similar ex- 

 ternal conditions of life, the organs adapted to these conditions 

 are not the same in both. To instance these several state- 

 ments : when no organs can be observed in the primitive streak 

 of the embryo, it resembles the zoophyte, in which nutrition 

 is performed by imbibition ; but we observe in addition that 

 the primitive streak extends into a membrane which becomes 

 the vascular area. If we attempt the comparison when the 

 body resembles a worm, in as much as it is cylindrical and has 

 no limbs for motion, the resemblance scarcely extends further. 

 The worm has rings and contractile bands for its motions, 

 whilst the embryo has neither ; and the simple tube, which re- 

 presents the heart in both, gives indications of a higher organiza- 

 tion in the embryo. If the worm resides in an aqueous medium 

 like the embryo, it respires by means of gills, the embryo by a 

 production of its abdominal tube — the umbilical vesicle (?), or 

 the allantois, or the placenta. At another period remarkable 

 apertures are observed, at regular distances, towards the head, 

 between the imperfectly closed abdominal laminae in the higher 

 embryos, in which they resemble some of the cartilaginous fishes. 

 But with the former the vessels that follow the arches do not di- 

 vide for any respiratory purpose, whilst in the latter they are the 

 respiratory vessels of the gills. If, in a still further stage of ad- 

 vancement, we compare the higher embryo with the turtle, we find 

 that in both the double heart is rendered virtually single, but for 

 very different purposes, and here the similarity is at an end*. 



In these analogies, therefore, we look in vain for that precision 

 which can alone support the inference that has been deduced. 

 Far rather do we infer gradations of original power, which 

 manifest their different energies at different epochs, under ex- 

 ternal conditions which may be similar according to a general 

 plan, the expression of each that is superadded modifying that 

 of all which preceded, and concurring with theirs to develop 



* Weber in Hildebrandt's Anatomy, vol. i. p. 125. 



