147 



The accompanying diagram, which illustrates the process of contraction of 

 a sarcomere, shows another feature of the contractile process to which I hope 

 to refer more fully in a later paper. When individual sarcomeres are 

 examined during their gradual passage into a contracted region of the fibre, it is 

 seen that, although the dark-staining material is rapidly accumulating at the 

 Krause's membranes, yet no appreciable shortening of the sarcomere takes place. 

 It is only when the whole of the movable hyaloplasm has accumulated in the region 

 of Krause's membrane, that the former spreads out sideways, greatly increasing 

 the diameter of the sarcomere, and automatically shortening it. Not till now, 

 indeed, do the sarcomeres appreciably shorten ; they forsake their cylindrical 

 shape, and become converted into short dice-box shaped structures (fig. 2, pi. xi.). 

 We recognize, in fact, a short, but yet appreciable, latent period of contraction. 

 The curious shape of contracted sarcomeres likewise accounts for the increase 

 in volume of doubly refracting material as observed by Engelmann. A glance 

 at fig. 2, pi. xi., will show that a considerable part of what would appear doubly 

 refracting in all but very thin sections, is in reality not occupied by doubly 

 refracting material at all, but is made up of the interfibrillar spaces. 



The development of muscle fibres is best examined in insects. The process 

 has been studied by a number of authors, and I shall refer to it only briefly 

 here, in order to show the fundamental difference in the nature of ordinary 

 striated muscle and wing muscle of insects. 



The following brief remarks refer to the development of the leg muscles 

 of insects, as I have observed it in a small chalcid wasp (Nasonia). The muscle 

 is developed from embryonic cells (myoblasts) which in the early pupa form 

 thick columns of cells in the several segments of the leg (fig. 12, pi. xii.). These 

 columns later break up into a large number of secondary columns, formed each 

 of a single row of cells, arranged one behind the other. Adjacent cell-walls 

 break down, and each column is now represented by a long syncytial column, 

 with numerous nuclei arranged in a chain along its central axis (fig. 13, pi. 

 xii.). An internal differentiation of each of these columns later occurs, and the 

 whole mass breaks up into minute longitudinal fibrillae (fig. 14, pi. xii.). 

 These fibrillae then differentiate into alternate dark and light parts, and the 

 disposition of them is such that they collectively form a double spiral within 

 the fibre. The wing muscles of insects, as I will show later, have a wholly 

 different mode of origin. 



Note on the Motor Nerve Endings on Striated Muscle Fibres. 



In the endings of different nerves on striated muscle fibres of the frog I 

 have observed a curious feature to which attention has not, so far as I am aware, 

 hitherto been drawn. If a partial side view of the end plate nuclei is obtained there 

 can generally be seen protruding from the lower part of each nucleus a minute 

 process. I cannot say whether it is the termination of one of the minute fibrils 

 of the nerve ending, which sometimes appear to enter the nuclei, or whether 

 it is a process formed directly from the nucleus. But of its existence there can 

 be no doubt, and such a process is always found to lie in close connection, not 

 with the muscle "striation," but with Krause's membrane (fig. 20, pi. xiv.) Even 

 ordinary muscle nuclei can, in side view, be clearly seen to have a direct con- 

 nection with Krause's membrane. This is clearly shown in fig. 21, pi. xiv., 

 taken from the leg muscles of a Myriapod Scutigera. The nuclei, here, occur 

 along the central axis of the fibre; and each nucleus is seen to be connected by 

 short processes with Krause's membrane. The example is taken, it should be 

 noticed, from a contracted region of the fibre; there has occurred an 



