SECTIONAL TRANSACTIONS.— D. 343 



rotation of the symphysis in post-natal stages are, however, very striking and, from a 

 comparison of the specimens available, it is evident that there is a retrogressive 

 change in this portion of the mandible, while the alveolar portion of the body and the 

 whole of the ramus are becoming progressively specialised and enlarged. 



Dr. P. D. F. Murray. — Factors in the Early Development of the Vertebrate 

 Skeleton. 



The mechanism of early skeletal development, with special reference to the avian 

 limb, is discussed in the light of two main series of experiments. In one series frag- 

 ments of limb buds of chick embryos which had been incubated for three, four and 

 five days were grafted upon the chorio-allantois of eight-day chicks and allowed to 

 live as grafts for from seven to nine days. In the second series the cartilaginous 

 femora were removed from six and seven day embryos, were cleaned of all soft parts 

 except the perichondrium, and were grafted in the same way. 



The main conclusions reached in the discussion are : 



(1) The development of the gross form of the cartilaginous skeleton is an affair 

 of factors intrinsic in the individual skeletal segments themselves, i.e. the gross 

 skeletal form self-differentiates. This is true of both shafts and epiphyses. Halves 

 of limb buds of four and five-day chicks, grafted on to the chorio-allantois, have 

 developed femoral shafts of practically normal form with normal curvatures, in the 

 absence of either pelvis or of tibio-fibula or of both. Cases have occurred in ^^•hich 

 femora have developed with nearly normal heads in the absence of the pelvis and of 

 nearly normal condyles in the absence of the tibia and fibula. * 



(2) Mechanical factors extrinsic to the skeletal segments, such as pressures and 

 tensions resulting from the growth of contiguous segments, from muscle pull, &c., 

 are not of primary importance as active factors in the development of skeletal form, 

 but are of great secondary importance in providing conditions suitable for the action 

 of the intrinsic factors and in bringing into existence the detailed perfection of form 

 required of the functioning skeleton. 



(3) The gross form of the bony skeleton, in the case of replacing bones, is pre- 

 determined by that of the cartilaginous skeleton. Certain facts suggest that an 

 important condition affecting early osteogenesis is a loosening of the perichondrium 

 around the middle of the cartilaginous shaft through the growth of the two epiphyses. 



Mr. G. L. Purser. — A Reconsideration of certain Embryonic Stages. 



Afternoon. 



Miss P. M. Jenkin and Dr. E. B. Worthington.^ — A Symposium on tJie 

 Ecology of African Lakes. 



Dr. E. B. WoRTHiNGTON. — Until the last few years biological work on the African 

 lakes has consisted largely of the collection and description of species within the 

 different lakes. The ecological side was hardly touched till the Government fishing 

 surveys of Lakes Victoria, Albert and Kioga in 1927-28. There are two methods of 

 working out the ecology in such lakes. One is to start at the top of the chain with the 

 large predaceous fishes and reptiles. By catching these in large numbers and examin- 

 ing their stomachs and gonads, their feeding and breeding habits are ascertained. 

 The same is done with their food, and so the ' food-chains ' in the lake are built up 

 link by link down to the phytoplankton and other vegetation. The other method is to 

 start at the bottom of the chain, to find out as much as possible about the physical 

 and chemical conditions, and to link up these with the plant life and so with the animal 

 life. Of course, both of these methods as well as quantitative collecting of the plankton, 

 nekton and benthos are necessary for a thorough ecological survey. 



The method of building up food-chains is well illustrated by the conditions in lake 

 Albert. At the top of the chain is the great Albert Perch (Lates albertianns) which ia 

 itself eaten only by human beings and crocodiles. It feeds on the Tigerfish {Hydrocyon) 

 and other fishes. The tigerfish eats small Hajjlochromis which eat shell-fish which eat 

 minute algae and detritus. Other food-chains lead to the phytoplankton and large 

 aquatic plants ; and aquatic tortoises, birds and mammals enter into the ecology also. 

 In Victoria and Kioga the food-chains are rather simpler owing to the absence of large 



