DEVELOPMENT OK \l\\: i \\ o MlHI'l.i; .il;\! I. \YER8. 1 -<> 



out farther laterally: it is connected wn li tin- origin of the middle 



germ layer, 



In a Mill later Mage of development (fig. 86), at the beginning of 



the second day of incubation, the lir-t fundament of tin- central 

 nervou- system make- its app aranco in the anterior |iortion of the 

 ir-im di-e. Over the head-proc-- me di-tance from 



ach other the two medullary fold- (.1). which are continuous with 

 each other at their anterior end-, and which hound the broad medul- 

 lary furrow (me); j)osteriorly they become less prominent, and they 

 here embrace between them the anterior end of the primitive streak 

 (j>r). Medullary furrow (,,ic) and primitive groove (pr) must not 

 be confounded with each other. a.- occurred in the earlier da\ 

 mbryolo:y ; they are two entirely distinct and disMmilar structure-, 

 which exiM at the same time, and independently of each other, as 



tig. SI) slnw>. 



I'l-imitive >treak and primitive groove are preserved for a long 

 time without unleririn.ir imp(rtant changes (fig. 87 pr). They 

 always occupy the posterior end of the embryonic body, which is 

 characterised by its .-lightly ditlerentiated condition even in BJ 

 when the development of the separate organs of the body i> a In ady 

 in full progress. On the contrary, the embryonic territory lyin.ir in 

 front of it, which is so small at the time of the appearance of the 

 head-process, becomes greatly elongated and, at the -a me time, 

 ditlerentiated into the >ej arate organs of the body. This process 

 of differentiation begins in front, and proceed- po-teriorly toward 

 the primitive groove, just as in Amphioxus and the Amphibia. 

 The margins of the medullary folds come into contact with each 

 other and berin to fuse, forming the neural tube. (Jib 1 , M-, }<l> : \ 

 ml'), the fusion jroi:re-sing from the head- toward the tail-end. 

 There are also to be recognised now in the interior of the body, 

 at either side of the neural tube, the protovertehne or primitive 

 .-enments (UN), which we shall inveMiirate more minutely further 

 on. The number of these i- con-tantly increa-ed by the growth 

 which is taking place at the tail-'-nd. 



When a large number of primitive segments ha- arisen, the 

 primiti\t> groove begins on sui'face-view.- to di.-appear : for it i.- >ur- 

 rounded l>y the medullary folds, and ii.a.-much as the-e fu-e he;. 

 well a.- elsewhere, it i- enclo-ed in the terminal part of the neural 

 tube. A notable condition, and one of great important- for the 

 interpretation of the primitive groove, ha- been di.-covered at this 

 etage m the embryos of several species d l-nd- by C^ASSKK. I'.KAIN, 



