838 REPORT—1890. 
early development of many insect embryos; and the depression of the Tzenia head 
within the cyst is a phenomenon of very similar nature. 
Restriction of the space within which development occurs often causes dis- 
placement or distortion of organs whose growth, restricted in its normal direction, 
takes place along the lines of least resistance. The telescoping of the limbs and 
other organs within the body of an insect larva is a simple case of such distortion ; 
and a more complicated example, closely comparable in many ways to the invagi- 
nation of the Tzenia head, is aflorded by the remarkable inversion of the germinal 
layers in Rodents, first described by Bischoff in the Guinea pig, and long believed 
to be peculiar to that animal, but subsequently and simultaneously discovered by 
three independent observers, Kupffer, Selenka, and Fraser, to occur in varying 
degrees in rats, mice, and in other rodents. 
One of the most recent attempts to explain developmental peculiarities as due 
to mechanical causes is Mr. Dendy’s suggestion with regard to the pseudogastrula 
stage in the development of the calcareous sponges. It is well known that while the 
larva is in the amphiblastula stage, and still embedded in the tissues of the parent, 
the granular cells become invaginated within the ciliated cells, giving rise to the 
pseudogastrula stage. At a slightly later stage, when the larva becomes free, the 
invaginated granular cells become again everted, and the larva spherical in shape ; 
while still later vagination occurs once more, the ciliated cells being this time 
invaginated within the granular cells, The significance of the pseudogastrula 
stage has hitherto been undetermined, but Mr. Dendy points out that the larva 
always occupies a definite position with reference to the parental tissues; that the 
ciliated half of the larva is covered by a soft and yielding wall, while the opposite 
half, composed of the granular cells, is covered by a layer stiffened with rigid 
spicules ; and his observations on the growth of the larva lead him to think that the 
pseudogastrula stage is brought about mechanically by flattening of the granular 
cells through pressure against this rigid wall of spicules. 
* Embryology supplies us with many unsolved problems, and it is not to be 
wondered at that this should be the case. Some of these may fairly be spoken of 
as mere curiosities of development, while others are clearly of greater moment. I 
do not propose to catalogue these, but will merely mention two or three which 
I happen to have recently run my head against and remember vividly. 
The solid condition of the cesophagus in Hlasmobranch embryos, first noticed by 
Balfour, is a very curious point. The cesophagus has at first a well-developed 
lumen, like the rest of the alimentary canal; but at an early period, stage K of 
Balfour’s nomenclature, the part of the oesophagus overlying the heart, and immedi- 
ately behind the branchial region, becomes solid, and remains solid for a long time, 
the exact date of reappearance of the lumen not being yet ascertained. 
Mr. Bles and myself have recently noticed that a similar solidification of the 
cesophagus occurs in tadpoles of the common frog. In young free swimming 
tadpoles the cesophagus is perforate, but in tadpoles of about 7} mm. length it 
becomes solid and remains so until a length of about 104 mm. has been attained. 
The solidification occurs at a stage closely corresponding with that in which it first 
appears in the dogfish, and a curious point about it is that in the frog the 
cesophagus hecomes solid just before the mouth opening is formed, and remains 
solid for some little time after this important event. 
This closing of the cesophagus clearly cannot be recapitulation, but the fact 
that it occurs at corresponding periods in the frog and dogfish suggests that it may 
possibly, as Balfour hinted, ‘turn out to have some unsuspected morphological 
bearing. 
Another developmental curiosity is the duplication of the gill slits by growth 
downwards of tongues from their dorsal margins ; a duplication which is described 
as occurring in Amphioxus and in Balanoglossus, but in no other animal; and the 
occurrence of which, in apparently closely similar fashion, is one of the strongest 
arguments in favour of a real affinity between these two forms. It is hardly 
possible that such a modification should have been acquired independently twice 
over. 
A much more litigious question is the significance of the neurenteric canal of 
Biss 
