VI. A COMPARISON OF THE EARLY STAGES IN THE DE- 

 VELOPMENT OF VERTEBRATES 1 . 



With Plate 5. 



IF the genealogical relationships of animals are to be mainly 

 or largely determined on embryological evidence, it becomes a 

 matter of great importance to know how far evidence of this 

 kind is trustworthy. 



The dependence to be placed on it has been general!}- 

 assumed to be nearly complete. Yet there appears to be no 

 a priori reason why natural selection should not act during the 

 embryonic as well as the adult period of life ; and there is no 

 question that during their embryonic existence animals are 

 more susceptible to external forces than after they have be- 

 come full grown : indeed, an immense mass of evidence could 

 be brought to shew that these forces do act upon embryos, 

 and produce in them great alterations tending to obscure the 

 genealogical inferences to be gathered from their developmental 

 histories. Even the time-honoured layers form to this no ex- 

 ception. In Elasuwbranchs, for instance, we find the notochord 

 derived from the hypoblast and the spinal ganglia derived from 

 the involuted epiblast of the neural canal, whilst in the higher 

 vertebrates both of these organs are formed in the mesoblast. 

 Such instances are leading embryologists to recognise the fact 

 that the so-called layers are not quite constant and must not be 

 absolutely depended upon in the determination of homologies. 

 But though it is necessary to recognise the fact that great 

 changes do occur in animals during their embryonic life, it is 

 not necessary to conclude that all embryological evidence is 

 thereby vitiated ; but rather it becomes incumbent on us to 

 attempt to determine which embryological features are an- 

 cestral and which secondary. For this purpose it is requisite 



1 From the Quarterly Journal of Microscopical Science, Vol. XV. 1875. 



