3 86 / ARTHUR THOMSON [may 



Beginning with the vascular system, which he regards as essentially 

 entoblastic in origin, and as originally derivable from a dorsal aorta 

 or pair of arteries and a sub-intestinal vein, connected by metameric 

 cross-vessels as in some Annelids, Mehnert maintains that the heart 

 is a secondary differentiation, arising as the result of functional 

 adaptation from a portion of the primary ventral vessel where the 

 greatest resistance had to be overcome. But when we turn to the 

 actual development of the heart, we find that in lamprey and dogfish, 

 newt and frog, chick and pig, rabbit and man, the heart is differen- 

 tiated before the associated main vessels. There is thus an obvious 

 lack of congruence between the ontogenetic or individual, and the 

 phylogenetic or racial, order of sequence, — a discrepancy which the 

 author deals with in his proposed corollary to the biogenetic law. 

 This corollary is, that as an organ rises in physiological importance 

 and structural differentiation, it acquires a proportionate increase in 

 its rate of development. In fact, as the author expresses it, the 

 embryological key to phylogenetic age is not to be found in the order 

 of appearance, but in the intensity of developmental energy. But what 

 this developmental energy precisely is we are not told. A second 

 illustration concerns the mammary glands. It is generally supposed 

 that these have arisen from more primitive integumentary glands of 

 less specialised type, such as the sebaceous or the sudorific glands. 

 The debated question which of these is the more nearly related to the 

 milk glands need not concern us now. What is striking is the fact 

 that the primordia of the mammary glands appear in man, for instance, 

 before there is any hint of sudorific or sebaceous glands. Again the 

 individual development seems to reverse that of the race ; again 

 Mehnert gives the interpretation that the acceleration of the mammary 

 glands is causally connected with their progressive physiological im- 

 portance and structural specialization. 



A third, and, as it seems to us, much less effective illustration is 

 found in the pineal eye, which Mehnert regards as historically older 

 than the paired eyes and possibly homologous with the eye of 

 Tunicates. But in all vertebrates hitherto studied embryologically 

 the paired eyes, if present, arise decidedly earlier than the pineal eye. 

 The organ which has lost its functional dignity, so to speak, is 

 handicapped in its rate of development. 



Communications between the cavity of the " protovertebrae " and 

 the cavity of the lateral plates are primitive in lower vertebrates like 

 the lancelet and selachians, but they are not seen in the early stages 

 of bird and mammal, yet Bonnet has described these communications 

 in four segments in a late embryo of the sheep (15-17 days), and 

 Dexter has made a similar observation in regard to the chick. This 

 late occurrence of a structural detail which is doubtless on the 

 retrograde path harmonises with Mehnert's view. Betrogression and 

 retardation go hand in hand. The late occurrence of a neurenteric 



