134 



Fin ST FORMATION OF THE EMBRYO. 



gives rise to the tissue wliicli eventually foruis the area vasculosa. It seems 

 probable that the mesoblast in this situation is mainly derived from cells 

 formed around the nuclei of the germinal wall, which are usually specially 

 aggregated close below the epiblast, Disse (No. 122) has especially brought 

 evidence in favour of this view, and my own observations also support it. 



The mesoblast ic somites begin to be formed in the lateral plates 

 of the mesoblast before the chjsure of the medullary folds. The 

 first somite arises close to the foremost extremity of the primitive 

 streak, but the next is stated to arise in front of this, so that the first 

 formed somite corresponds to the second permanent vertebra'. The 

 region of the embryo in front of the second formed somite — at 

 first the largest part of the embryo — is the cephalic region. The 

 somites foUowintr the second are formed in the regular manner, from 

 befoi-e backwards, ovit of the unsegmented posterior part of the 

 embryo, which rapidly grows in length to supply the necessary 

 material (fig. 103;. As tlie somites retain during the early stages 

 of development an approximately constant breadth, their number 

 is a fair test of the length of the trunk. With the growth of the 

 embryo the primitive streak is continually carried back, the length- 

 ening of the embryo always taking place between the front end 

 of the primitive streak and the last somite ; and during this process 



the primitive streak undergoes important 

 changes both in itself and in its relation 

 to the embryo. Its anterior thicker part, 

 which is enveloped in the diverging me- 

 dullary folds, soon becomes distinguished 

 in structure from the part behind this, and 

 placed symmetrically in relation to the 

 axis of the embryo (fig, 103, a.pr), and at 

 the same time the medullary folds, which 

 at first simply diverge on each side of the 

 primitive streak, bend in again and meet 

 behind so as completely to enclose the front 

 part of tlie primitive streak. The region of 

 the embryo bird, where the medullary folds 

 diverge, is known as the sinus rhomboid- 

 |ilis, though it has no connection with tTie 

 similarly named structure in the adult. By 

 the time that ten somites are formed the 

 sinus rhomboidalis is completely established, 

 and the medullary groove has become con- 

 verted into a tube till close up to the front 

 end of the sinus. In the following stages 

 the closure of the medullary canal extends 

 to the sinus rhomboidalis, and the foldinsr 



Fio. 103. Dorsal view of 



THE HARDENED BLASTODERM OP 

 A CHICK WITH FIVE MESOBLAS- 



Tic SOMITES. The medullary 



FOLDS HAVE MET FOR PART OF 

 THEIR EXTENT, BUT HAVE NOT 

 UNITED. 



a.jn-. anterior part of the 

 primitive streak; p.pr. pos- 

 terior part of the primitive 

 streak. 



1 Further investigations in confirmation of this widely accepted statement are very 

 desirable. 



