2i6 EMBRYOLOGY IN THE SEVENTEENTH [pt. ii 



but it belongs to the present period, and I shall not treat it historically. 

 For data on von Baer, see Kirste, Addison and Stieda. It is interesting 

 to note, however, that the recapitulation theory, which was first clearly 

 formulated by von Baer, was already taking shape in various minds 

 during the closing years of the eighteenth century. Lewes has thus 

 described the thesis of Goethe's Morphologie, written in 1795: "The 

 more imperfect a being is the more do its individual parts resemble 

 each other and the more do these parts resemble the whole. The more 

 perfect a being is the more dissimilar are its parts. In the former case 

 the parts are more or less a repetition of the whole, in the latter 

 case they are totally unlike the whole. The more the parts resemble 

 each other the less subordination is there of one to the other : and 

 subordination is the mark of high grade of organisation". 



William and John Hunter belong also to the end of the century. 

 The former, in his book on the anatomy of the gravid uterus, proved 

 finally and completely the truth of the view that the maternal and 

 foetal circulations are distinct. His injections left no shadow of doubt 

 about the matter, and the way was clearly opened up for the study 

 of the properties of the capillary endothelial membranes separating 

 the bloods, a study which is still vigorously proceeding, especially 

 in its physico-chemical aspect (see Section 21). There was a quarrel 

 between the brothers over the priority of this demonstration. John 

 Hunter's Essays and Observations also contain material important for 

 embryology. His drawings of the chick in the &gg were very beautiful, 

 and are still in the archives of the Royal College of Surgeons. He 

 adopted Mayow's theory of the office of the air-space, and anticipated 

 von Baer's theory of recapitulation much as did Goethe. "If we were 

 capable of following the progress of increase of the number of parts 

 of the most perfect animal as they were first formed in succession, 

 from the very first to its state of full perfection, we should probably 

 be able to compare it with some one of the incomplete animals 

 themselves, of every order of animals in the creation, being at no 

 stage different from some of the inferior orders. Or, in other words, 

 if we were to take a series of animals, from the more imperfect to 

 the perfect, we should probably find an imperfect animal corre- 

 sponding with some stage of the most perfect." It is impossible not 

 to reflect on the curious course which was taken by the essence of 

 the idea of recapitulation in the history of embryology. As Aristotle 

 first formulated it, it was as much bodily as mental, but all his sue- 



