108 A. M. PATERSON. 



further changes are taking place, but in which the muscle-plate takes 

 no part. It remains within the body cavity, and ends below at the 

 same limit as before (Fig. 12, m.p.). Structurally it is the same. In 

 the limb buds themselves the first changes occur at this date, in the 

 formative blastema, which result in the production of the muscular 

 and osseous systems. The nerve plexuses, as we shall see below, 

 have been produced ; and the resulting trunks have passed into 

 the limb, in two groups, one dorsal (d.), the other ventral (v.). 

 Between, above, and below the nerves, the mesoblastic cells are 

 taking on a characteristic arrangement. The cells immediately 

 above and below each trunk (1, 2) are more closely packed together, 

 forming thick layers, each several cells deep. They are histologically 

 the same as before. In the centre of the limb bud (3), between the 

 nerve trunks, the cells are now arranged in a concentric and sym- 

 metrical fashion, and are separated from one another by a small 

 amount of intercellular substance. Towards the dorsal and ventral 

 surfaces of the limbs the cells are more scattered, so that the central 

 portion of the limb in transverse sections appears darker than the 

 superficial parts. At the free end of the limb (4) there is still no 

 differentiation of tissue elements. The cells here form a simple 

 mass, without any distinction into layers or groups. 



In a five days' embryo the condition of the muscle-plate in the 

 region between the limbs (Fig. 13, m. p.) is much the same as at the 

 last mentioned period. It shows still further extension in a ventral 

 direction down the body wall. In the same embryo, in the regions 

 of the limbs the muscle-plate (Fig. 14, m. p.) has clearly no connec- 

 tion with the muscular system of the limb itself. It consists now of 

 elongated fibreSj forming, in transverse sections, a column lying just 

 outside the spinal cord and nerves, and separated from the surface of 

 the body and from the limbs by a considerable thickness of ordinary 

 blastema. The several tissues of the limb are formed from mesoblastic 

 elements, developed in situ. A central core of cellular cartilage (3) is 

 very evident at this date. The cells are arranged regularly in a con- 

 centric manner in transverse sections ; in transverse rows in longitu- 

 dinal sections. This cartilaginous cylinder is found in longitudinal 

 sections to be broken up into segments, corresponding to the skeletal 

 elements of the limb. The intercellular substance between the 

 cells has largely increased in amount. At the periphery of this 



