18 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
embryo may be more tban one-balf larger than another of the same stage, 
and one with a single protovertebra may be as long as another with 
twelve. His results, taken along with those of Bonnet, Keibel, Mehnert, 
and Oppel, lead him to conclude that the variations in size also repre- 
sent differences in the internal structure of the parts. 
There seems to be no constant relation between the total length and 
the length of individual regions. In other words, proportions vary 
widely. Variations are commonest in the youngest stages, and decrease 
as differentiation increases. The correlations of the developing organs 
seem to have a regulating influence, narrowing the limits of variations in 
size and proportions. 
Fischel agrees generally with His, that absolute and relative growth 
in younger and older stages vary inversely, the former being greatest at 
first, but gradually decreasing. He adds that the intensity of growth 
varies with the individual, though there is always a relation of depend- 
ence between the intensity of relative growth and the total length of the 
embryo. Zones of intense growth are distinguishable, and the growth 
Foth of the whole length and of the several regions is periodic. A 
• “ plus ” of growth-energy in one region seems to imply a “ minus ” in 
another. The reasons for this remain obscure. 
Tooth-Genesis in Canid se.* — Hr. H. W. Marett Tims, in treating of 
the tooth-genesis in the Canidse, tells us that his main object was to 
trace the order of cusp-development, and the interrelationships of the 
various cusps in the tooth of the dogs, and to examine into the evidence, 
thereby obtained, bearing upon important and interesting problems of 
phylogeny. The author has been led to profound disagreement with 
Prof. Osborn, and he thinks that there are still other objections which 
may be urged against the tritubercular theory. These we need not sum- 
marise here. Suffice it to say that neither the tritubercular theory nor 
the multitubercular theory satisfies the author, and he proposes instead 
the theory of “ cingulum cusp-development.” This theory, we are told, 
is supported both by palaeontological and embryological evidence. 
The following are said to be the points in its favour : — (1) It har- 
monises more fully with what is known of the development of the teeth 
than either the tritubercular or multitubercular theory, the primary 
cone representing the reptilian cone and being always present. (2) It 
is quite possible and easy thus to homologise the cusps of all teeth, 
except perhaps the derivatives of the multituberculate type. (3) It is 
in accordance with palaeontological history. (4) No supposed rotation 
of cusps is required. 
Dentition of Manatee.t — Prof. W. Kiikenthal has studied an embryo 
of Manatus latirostris G*85 cm. in length, and establishes as the dental 
3003 
formula of the embryo — • In the course of phylogeny, reductions 
took place first in the upper jaw, canines and premolars disappearing, 
while the incisors became rudimentary sooner than those of the lower 
jaw. The teeth of the lower jaw retained their function longer, but also 
became useless; the third premolar was the first to disappear; there- 
* Journ. Linn. Soc., xxv. (189G) pp. 445-80 (8 figs.), 
f Anat. Anzeig., xii. (1896) pp. 513-26 (10 figs.). 
