114 A. M. PATEESON. 



through a third of the length of the body wall, lying close to 

 the outer side of the body cavity. Its relations and structure 

 are the same as before. Each end is surmounted by a cap of 

 cells, which retain their primitive characters ; rounded, fusi- 

 form, or angular, they stain deeply, and are plainly separated 

 off from the main part of the muscle-plate. These cells 

 can be traced on to the outer surface of the muscle-plate, 

 where they gradually become lost. The main part of the plate 

 consists of elongated fusiform cells. In the region of the fore 

 limbs (fig. 8) the mesoblastic tissue of the limb still presents 

 the same characters. The cells stain deeply, are round, ovoid, 

 and often multinucleated; but still undifferentiated. The 

 foetal vessels are better marked. The muscle- plate (m.p.) 

 occupies its primitive position, ending below at the root of the 

 limb. It has the same structural characters as in the dorsal 

 region ; but the undifferentiated cells, which stain deeply, are 

 best marked at the upper end of the plate. 



In embryos at four days twelve hours, in the trunk between 

 the limbs, the muscle-plate (fig. 11, m.p.) has passed half way 

 down the body wall, lying close outside the cavity. It now 

 consists almost entirely of elongated spindle-cells. In the 

 regions of the limbs 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 symmetrical fashion, and are 



