PREFACE. xxi 



every species is such ab initio, and takes its own course to the full 

 manifestation of its specific characters, agreeably with the nature 

 originally impressed upon the germ. 



A perch, a newt, a dog, a man, does not begin to be such only 

 when the embryologist may discern the dawnings of their respect- 

 ive specific characters. The embryo derived its nature, and the', 

 potency of self-development according to the specific pattern, from 

 the moment of the impregnation ; and each step of development 

 moves to that consummation as its end and aim. 



This truth has been masked to some apprehensions by the 

 course of the developmental steps from the general to the par- 

 ticular; the initial ones, more especially, offering likenesses or 

 analogies to finished lower species exemplifying degrees of organi- 

 sation in the animal kingdom. Each step differs in degree of 

 difference from the analogous grade at which a lower species rests, 

 and inversely as the advance of such species. Accordingly, the 

 less the degree of difference, and the wider the resemblance or 

 analogy spreads between the embryonal phase and the parallel 

 grade in the series of species. 



The formation of the germ-mass (Vol. I. figs. 1-4, 422,452) — 

 the first step after impregnation — is a general phenomenon in 

 animality (Vol. ' On Invertebrates,' figs. 48-56, 73, 74, 80-84, 

 181, 209-212, 232) ; thereat and thereby the man resembles and 

 behaves like the monad. 1 But, the germ-mass completed, the 

 vertebrate at once circumscribes itself or withdraws into its 

 vertebrality. The proteine substance is the seat of a chemical 

 differencing, leading to excess of albumen along one tract, 

 balanced by excess of gelatine along a parallel tract. Thus are 

 laid clown the bases of the myelencephalon and vertebral axis. 

 The ' notochord ' is soon followed by the protovertebral specks 

 in double parallel series (Vol. I. fig. 5; Vol.11, fig. 133): the 

 embryonal trace is established, and it is one of a vertebrate. 



The formation of neural and hcemal arches next follows ; and 



1 Compare the above-cited figures with fig. \7. ' Lectures on Invertebrates,' 2nd 

 ed. p. 29. 



