D.—ZOOLOGY. 115 
the development and application of biochemical methods will indefinitely 
_ increase the weight of this testimony, but the main thesis appears to be 
established, namely, that there is direct evidence of the presence of a 
primary metabolic gradient along the major axis of the body. 
The indirect evidence is more easily appreciated by the general body of 
zoologists, and it is of the greatest interest. If the value of a hypothesis 
consists in the number of phenomena that are subsumed under it, then the 
gradient hypothesis on morphological evidence alone may take high rank. 
Old-established facts acquire new meaning. 
The general succession of cellular events in animal development shows 
that the fertilised egg has a radial or bilateral symmetry before it exhibits 
cell division. Normal and experimental evidence point clearly to the 
conclusion that the first act of morphogenesis is the establishment in most 
animals of the head end, and in Coelenterates of an apical region. This 
is followed by the development of the dorsal surface in Vertebrates, and of 
the ventral surface in most Invertebrates, determining in each case the 
_ foundations of the nervous system. Simultaneously the lateral organs are 
_ laid down usually in the form of ‘ segments,’ the outer part of which remains 
more embryonic and plastic, whilst the inner part, abutting on the axis of 
the embryo, undergoes more rapid and elaborate morphogenesis. The 
whole process of the gradual establishment of the primary rudiments of 
bodily structure in the embryo is not only consistent with the theory of 
gradients, but receives (perhaps for the first time) a rational ‘ explanation.’ 
(B) Regeneration. 
Perhaps even more suggestive than the facts of individual development 
are the conclusions of experiment, both natural and artificial, upon the 
regeneration of animal organs and tissues. The main facts as to the 
extent and occurrence of the faculty for renewal of lost parts by animal 
tissues are well known, and need not be traversed here, but there are some 
special cases that are little known, and that form a test of the validity 
of the gradient hypothesis. Moreover, as this view grew out of the con- 
sideration of data given by the regeneration of animals, it is appropriate 
that this large body of analytical work should receive mention. 
Child’s work, and that of his pupils, has shown that in certain freshwater 
Planarians, only experimental difficulties set a limit to the minimal quantity 
of the body that will regenerate the whole. If and when these difficulties 
are overcome, it is probable that a single isolated cell of many of the lower 
animals may be induced toregeneratethe whole, as is the casein many plants. 
We are only at the threshold of these inquiries, and the progress of tissue- 
culture, which is now being actively pursued, will undoubtedly open up 
_ hew ranges of control over the technique of physiological isolation. It will 
_ be remembered that H. V. Wilson and J. S. Huxley (7) have shown that 
from the artificial fragmentation of a sponge or hydroid, new individuals 
arise. From a few of those fragments—sheddings composed of cell- 
_ groups, and even a few isolated cells placed in suitable conditions—there 
arises by cellular conjunction a small amorphous mass, which acquires 
polarity and differentiation, and forms a new sponge or hydroid, recalling 
the reconstitution of ‘ an exceeding great army ’ in Hzekiel’s vision of the 
valley of dry bones. We seem driven to the conclusion that every cell 
of these animals only develops a portion of its potentiality when actively 
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