22 PROFESSOR A. M. MARSHALL. 



to mechanical causes, and may fairly be considered under the head of 

 developmental conveniences 



The amnion of higher vertebrates is a case in point, and is probably 

 rightly explained as due in the first instance to sinking or depression of 

 the embryo into the yolk, in order to avoid distortion through pressure 

 against a hard unyielding eggshell. A similar device is employed, 

 presumably for the same reason, in the early development of many 

 insect embryos ; and the depression of the Taenia head within the cyst 

 is a phenomenon of very similar nature. 



Restriction of the space within which development occurs often 

 causes displacement 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 invagination of the 

 Taenia head, is afforded 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 simul- 

 taneously 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 imbedded 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 invagination occurs once more, the ciliated 

 calls being this time invaginated within the granular cells. The 

 significance of the pseudosgastrula stage has hitherto been undeter- 

 mined, 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 



