1Q6 DE. WILSON FOX ON THE DEVELOPMENT 



these bodies, as shown in figs. 29 & 30 (Plates V. & VI). They have become much elon- 

 gated, forming long fibres, in which both a longitudinal and often a transverse striation 

 are apparent ; and towards the centre of the long axis of each of these fibres appears the 

 nucleus, external to which is a distinct membrane, which can often be seen prolonged 

 upon the fibre for some distance. These fibres form a dense tissue, the enlargements 

 opposite the nuclei fitting into the narrower portions of adjacent fibres. Sometimes 

 intermediate stages are seen between these and the earlier spindle-shaped cells (as in 

 Plate V. fig. 28). 



From the fourth to the fifth day a further change in the fibres is observed, which 

 consists in the multiplication of the nuclei in their interior (see Plate VI. figs. 31-34). 



At this stage the presence of a membrane enclosing the nuclei is very apparent. 

 The nuclei form groups of two, three, four, or more in number, and vary somewhat 

 in size. They are seen to have very distinct nucleoli, and are surrounded on all sides, 

 except where they lie in apposition with the striated portion, with a granular nebulous 

 matter, which may often be seen contained under the membrane, surrounding 

 the narrower portion of the fibre. Figs. 31 & 32 (Plate VI.) are profile and front 

 views of fibres, with a group of nuclei situated upon them and enclosed by a mem- 

 brane which, though of considerable tenuity, is sufficiently defined to give a sharp 

 double contour to the fibre. At fig. 34 (Plate VI.) the nuclei may be seen 

 lying in various positions on the fibre, enclosed by a very distinct membrane, which at 

 a is thrown into folds across the surface of the fibre. After the fifth day the multi- 

 plication of the nuclei proceeds to a much greater extent, and is attended with an 

 increase in thickness of the striated portion of the fibre. Thus figs. 36 & 37 (Plate 

 VI.) represent fibres from a Chick at the seventh and eighth days, as seen in 

 front view and profile, and in which the conditions are similar to those previously 

 observed, with the exception of such difiierences as arise from the increase in thickness 

 of the fibre. The continuity of the external membrane is very noticeable, especially 

 when it extends over the bulgings created by the groups of nuclei. 



I have found the process of development in the Sheep to proceed in precisely the 

 same manner as in the Tadpole and Chick. Figs. 38-44 (Plate VI.) represent 

 forms which I have seen, and which are almost precisely identical with those seen in 

 the Chick. The process of early growth can be' best observed in the budding extre- 

 mities of embryos of half an inch in length ; but the multiplication of the nuclei, as 

 seen in fig. 44, proceeds until the embryo is 4 or 5 inches long. 



The earliest period at which I have examined the human embryo was at six weeks. 

 I then found structures like figs. 45 & 46 (Plate VI.), which will be seen to 

 correspond in all particulars to figs. 18 & 19 (Plate V.), from the Frog, figs. 

 29 & 30, from the Chick, and figs. 39 & 40, from the Sheep. I have not succeeded in 

 obtaining embryos between the sixth week and the third month. By the latter period 

 the muscles have for the most part acquired their adult structure, though the fibres 

 are very much narrower. 



