514 miss joan b. procter on the 



Discussion. 

 The morphogeny of the carapace and pilastron. 



The primitive plastron, tberefore, and the delicate network of 

 dermal bone which forms the carapace of the adult Testudo 

 lovei'idgii, are brought about by arrested development. The 

 fenestrse have not been formed in the completed structures by 

 absorption, and do not increase in this manner with age as 

 supposed. The relative thickness or breadth of the plates depends 

 on individual variation, specimens of abnormal fragility having 

 had their development in this respect cut short at an earlier 

 stage than the average individual. This principle also applies 

 to such species as Testudo jyolyphemus, in which the carapace is 

 said to become thin and form fenestrse with age. A specimen 

 in the British Museum Collection of 210 mm. (76. 1. 36. 6) and a 

 half-grown specimen of 135 mm. (73.8.13.25) have both 

 carapaces and plastrons fenestrated in a manner similar to those 

 of T. loveridgii, whilst one of 225 mm. has them thin but entire. 



The ribs of the latter on the other hand, and seemingly the 

 neural arch of the vertebr?o, do absorb with growth ; the ribs 

 completely in most cases. Thus the bony plates are developing, 

 whilst the ribs are degenerating by osteoclnstic absorption — a 

 process which is, of course, physiological, and in fact, as far as 

 the ribs are concerned, carries to a logical conclusion a tendency 

 which is well marked throughout the genus. 



The question which now arises is : what has led to the extra- 

 ordinary degree of depression of the carapace ? Some individuals 

 are positively concave above, and all have a markedly deformed 

 and rickety appearance. 



Flattening is evidently not brought about, at any rate to this 

 extent, by a fenestrated or thin condition of the carapace, for the 

 giant tortoises which pass tlirough these stages in the course of 

 development and have very thin carapaces, are remarkably convex 

 in form. The only visible result here is the crumpled appearance 

 of the shell of T. ahingdonii. T. polyphemibs, on the other hand, 

 is distinctly flattened. 



It can be argued on the one hand that the flattened carapace is 

 brought about by the habit of living beneath stones and squeezing 

 into rock-crevices. This habit, induced by environment, would be 

 bound to have a modifying efl^ect ; for, during youth, the develop- 

 ment of a domed and solid carapace would be interfered with hj 

 the constant application of pressure, and in a sufficient number 

 of generations the ability to form a normal carapace might be lost 

 altogether. The fact that the Burrowing Tortoise, T. j^olyphemus, 

 has a thin or fenestrated and somewhat flattened carapace 

 supports this view. Could this be proved experimentally, it 

 woidd furnish a convincing argument in favour of the heritance 

 of acquired characters. 



On the other hand, it can be equally well maintained 

 that an inherited tendency to the arrest in development is 



