THE PRINCIPLES OF EMBRYOLOGY 463 
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 
