188 K. S. Beiji;h on the Development of the 



XXVIII. — O71 the Devclo/'inefit of (he Qerminal Streak of 

 Mysis. By R. IS. Bi:iuiii, of Copenhagen *. 



My investigations commence a])])roxiniately at the stage at 

 Avliich tlie segmentation has eonehuled and tlie formation of 

 the germinal hiyers begins. On the termination of the process 

 of segmentation the bhastoderm has extended ronnd tlie yolk 

 in every direction, and there now arises a thickening of the 

 blastoderm in the form of a transverse streak (this streak 

 occuj)ies a transverse position with reference both to the longi- 

 tudinal axis of the ovum as also to the subsequent longitudinal 

 axis of the embryo). In this thickened streak the cells are at 

 first arranged in only a single layer, and the thickening is 

 consequently occasioned merely by the greater height of the 

 cells of this regioTi. Soon, however, the streak becomes 

 Lilaminate at a certain spot in the neighbourhood of the 

 median line, since some few cells push their way inwards and 

 shortly afterwards undergo active multiplication within the 

 outer layer. I have observed stages in which only two, four, 

 or six iimer cells are present, but the number soon becomes 

 much larger; figures of nuclear division arc frequently met 

 •VN'ith. The inner mass of cells which has thus been formed 

 now speedily separates into three ditfcrent rudiments : — 

 (1) certain cells wander about and develop into vite//oj>ha(/a 

 (** Vitelloj)hagen ") ; (2) other cells (which adjoin the median 

 line) become more firmly united together into a plate and 

 constitute the true (intestinal) endoderm ; (3) towards the 

 sides certain others develop as 2>>•///i/<^'^;<3 cells of the muscle- 

 plates (mesoderm of authors). The final number of these 

 primitive cells is four on each side, though in earlier stages 

 not so many of these larger lateral cells are to be seen. As 

 soon as the definitive number is reached these cells commence 

 to jiroduce smaller ones in front by the process of budding 

 (" Knospung ") ; thus four longitudinal rows of cells are 

 formed on each side within the ectoderm, so that in certain 

 stages any transverse section from the region in question 

 contains tour muscle-plate cells on either side. With further 

 growth the muscle-jjlates become very distinctly segmented, 

 and 1 sec no reason to doubt that tlieir divisions correspond to 

 actual proto-segments. The latter soon become se[)arated 

 from one another, since the growth of the muscle-plates does 



♦ Trauslated from fbe ' ZoologisclitT Anzeigcr,' xv. Jahrg., no. 400, 

 November 28, 181(2, pp. 43(3-440. 



