THE PRINCIPLES OF EMBRYOLOGY 46 



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in which these later stages arise from the blastula stage ; here, as in 

 all cases, the ontogenetic laws must be in harmony with the phylo- 

 genetic ; of the latter the most important is the steady develop- 

 ment of the central nervous system for the upward progress of the 

 animal race. The study of comparative anatomy indicates the central 

 nervous system, not the gut, as the keystone of the edifice. So, also, it 

 must be with ontogeny ; here also the central factor in the formation 

 of the adult from the blastula ought to be the formation of the 

 central nervous system, not that of the gut. 



Such, it appears to me, is the case, as may be seen from the 

 following considerations. 



The study of the development of any animal can be treated in 

 two ways : either we can trace back from the adult to the very 

 beginning in the ovum, or we can trace forward from the fertilized 

 egg to the adult. Both methods ought to lead to the same result ; 

 the difference is, that in the first case we are passing from the more 

 known to the less known, and are expressing the unknown in terms 

 of the known. In the second case we are passing from the less 

 known to the more known, and are expressing the known in specula- 

 tive terms, invented to explain the unknown. What has just been 

 said with respect to the germinal layers means that, however much 

 we may study the embryo and try to express the adult in terms of it, 

 we finally come back to the first way of looking at the question, and, 

 starting with the adult, trace the continuity of function back to the 

 first formation of cells having a separate function. 



Let us, then, apply this throughout, and see what are the logical 

 results of tracing back the various organs and tissues from the adult 

 to the embryo. 



The adult body is built up of different kinds of tissues, which fall 

 naturally, from the standpoint of physiology, into groups. Such 

 groups are, in the first place — 



1. All those tissues which are connected with the central nervous 



system, including in that group the nervous system itself. 



2. All those tissues which have no connection with the nervous 



system. 



In the second group the physiologist places all germinal cells, all 



blood- and lymph-corpuscles, all plasma-cells and connective tissue 



and its derivatives — in fact, all free-living cells, whether in a free 



state or in a quiescent, so to speak encysted, condition, such as is 



