34 SUMMARY OF CURRENT RESEARCHES RELATING TO 



which he calls " luxees." The bones of the second sesimenfc of the 

 hind leg are delicate and short ; the femoro-tibial articulation is very 

 loose ; the animal moves on the distal end of the femur; the rest of the 

 limb is turned upwards. These peculiar forms bred true among them- 

 selves, and when crossed with normal forms yield offspring normal and 

 dislocated in the 3 : 1 proportion. In short, the peculiarity is recessive. 

 But after some months it was observed that a pair of dislocated mice 

 gave rise to a normal type. Seventeen normal forms were obtained 

 from eight recessive pairs. These behave as dominants, but besides 

 normal offspring they yielded forms dislocated on only one side. When 

 they were paired with normal forms they behaved as recessives. When 

 paired with bilaterally dislocated forms they showed a relative reces- 

 sivity. When inbred they yield unilaterally dislocated, bilaterally dis- 

 located and normal forms. The author indicates some of the difficulties 

 in using the factor hypothesis in interpreting his observations. He 

 inclines to regard the mutation as a quite novel physico-chemical change 

 in the constitution of gametes, probably induced by some peculiarity in 

 the amphimixis. 



HaBmopoiesis in Mongoose Embryo.* — H. E. Jordan has studied 

 the development of the blood in the yolk-sac and in intra-embryonic 

 mesenchyme. The mesenchyme is a fundamental htemogenic tissue. 

 It produces (by mechanical differentiation) endothelium and meso- 

 thelium, which retain its differentiative capacity. In early stages both 

 give rise to hsemoblasts. In the young embryo these ha^moblasts 

 differentiate into erythrocytes. The htemoblasts seem to be identifiable 

 with lymphocytes or primitive leucocytes, which are regarded as pro- 

 genitors of granulocytes. If this be so, it follows that the primitive 

 leucocytes appear before the erythrocytes. Such an ontogenetic 

 sequence is in accord with the principle of progressive differentiation 

 and with the phylogenetic history. It will be seen that Jordan strongly 

 supports the monophyletic theory of blood-cell origin. 



Aortic Cell-clusters in Vertebrate Embryos.f — H. E. Jordan 

 discusses in a variety of cases (embryos of mongoose, turtle, etc.) these 

 endothelial derivatives with hsmogenic significance. They illustrate 

 the inherent capacity of endothelium to produce hsemoblasts. " The 

 explanation of tlie limited distribution of the clusters is to be found in 

 a relationship to young or newly formed, only slightly differentiated, 

 epithelium, rather than in a connexion with regressive blood-vessels 

 and an associated toxic substance." 



Development of Mammary Glands.:!:— J. A. Myers describes the 

 foetal development of the mammary gland in the female albino rat (litis 

 tiorvegicus albinus). The early development has been previously de- 

 scribed by Henneberg, and the post-natal development by Myers. In 

 f tt'tuses at fifteen days and nine hours the glands are in the club-shaped 



* Carnegie Inst. Washington, Publication No. 251 (1917) pp. 291-312 (4 pis.). 



t.Proc. Nat. Acad. Sci., iii. (1917) pp. 149-56. 



J-Amer. Journ. Anat., xxii. (1917) pp. 195-222 (12 figs.). 



