Limb and Girdle 



435 



of the posture assumed by the free extremity. 

 In the diagram (Fig. 154) the nearly 

 central region of the limb area (between 3 

 and 5) is comparatively free of girdle-form- 

 ing cells. Gradually the limb posture be- 

 comes dominated by the girdle structiires at 

 its proximal extremity. This is shown by 

 experiments in which the developing tissues 

 around the limb area are rotated, producing 

 a corresponding rotation of the limb without 

 reversing its laterality. In this process of 

 postural control the outlying rudiments must 

 presumptively be considered as the most 

 effective factors. When the limb is trans- 



tated the girdle may, instead of showing 

 its normal morphological components, de- 

 velop as a plate-like sheet of cartilage. 



Since the forelimb mesoderm will grow 

 into a forelimb under various experimental 

 conditions, including the inversion of the 

 mediolateral axis, why is it that a limb 

 disc does not always develop two limbs, 

 as experimentally demonstrated by Nieuw- 

 koop ('46) in embryos deprived of the 

 yolk mass? It is, of course, easy to assume 

 a simple mechanical inhibition caused by 

 the presence of the heavy yolk. When, how- 

 ever, a small bit of head ectoderm is placed 



Fig. 154. The various portions of the girdle-forming region have been tested by transplantation experi- 

 ments for which the parts marked out by the vertical lines (a to e) and the horizontal lines (i to 7) 

 were employed singly or in combination (from Detwiler, '18; Swett, '23; and Nicholas, unpublished). 



planted heterotopically with but a small part 

 of the scapula, the imparted rotation is not 

 corrected in later stages. 



The relation between the girdle and the 

 free extremity is important also in redupli- 

 cation, reversal of symmetry and inversion. 

 The work of Swett ('32, '45) throws some 

 light on these problems but much remains 

 to be explained. Usually the girdle is ab- 

 normal when the limb is duplicated after 

 operations involving rotations of various de- 

 grees. Usually after orthotopic inverted limb 

 disc transplantations the limb is reversed in 

 its asymmetry and so is its girdle, i.e., a 

 right limb disc upside down on the right 

 side of the embryo becomes a left limb with 

 a left-sided girdle. This speaks strongly for 

 a change in the mosaic structvire of the gir- 

 dle parts, for their localized developmental 

 potencies cannot be irreversibly fixed or 

 determined. When areas as large as 5 

 somites in diameter are used for transplan- 

 tation there is seldom formed a reduplicat- 

 ing limb, whether orthotopically or hetero- 

 topically transplanted. If this graft is ro- 



over the limb mesoderm, replacing its nor- 

 mal covering ectoderm, a similar result is 

 obtained, and here the mechanical effect can- 

 not be assumed to be perceptibly greater 

 than in the normal ectoderm. Yet there is 

 inhibition of differentiation. The presence of 

 the developing girdle might also be thought 

 of as a possible factor that causes the ex- 

 tremity to develop outside the flank instead 

 of inside the coelom, but it lacks the ability 

 to make the mesoderm form a free extremity 

 when covered with head ectoderm. There 

 are still many problems of morphogenesis to 

 which the limb-girdle combination may con- 

 tribute solutions. 



THE PELVIC GIRDLE 



Stultz ('36) has performed a series of ex- 

 periments which yield information regarding 

 the development of the hindlimb and girdle 

 of Ambly stoma. The same course of events 

 is followed as in the forelimb but the mo- 

 saic constitution of the girdle is not so 

 striking as in the forelimb. In his analysis 



