406 IN SECT A. 



Hatschek (No. 414), and Graber (No. 412), etc. From their 

 researches it would appear that the formation of the mesoblast 

 always takes place in a manner closely resembling that in 

 Hydrophilus. The essential features of the process (figs. 177 

 and 178) appear to be that a groove is formed along the median 

 line of the ventral plate, and that the sides of this groove either 

 (i) simply close over like the walls of the medullary groove in 

 Vertebrates, and so convert the groove into a tube, which soon 

 becomes solid and forms a mass or plate of cells internal to the 

 epiblast ; or (2) that the cells on each side of the groove grow 

 over it and meet in the middle line, forming a layer external 

 to the cells which lined the groove. The former of these 

 processes is the most usual ; and in the Muscidae the dimensions 

 of the groove are very considerable (Graber, No. 411). In both 

 cases the process is fundamentally the same, and causes the 

 ventral plate to become divided into two layers 1 . The external 

 layer or epiblast is an uniform sheet forming the main part of 

 the ventral plate (fig. 178 B, ep). It is continuous at its edge 

 with the amnion. The inner layer or mesoblast constitutes an 

 independent plate of cells internal to the epiblast (fig. 178 B, me). 

 The mesoblast soon becomes divided into two lateral bands. 



The origin of the hypoblast is still in dispute. It will be 

 remembered (vide pp. 114 and 1 16) that after the segmentation a 

 number of nuclei remain in the yolk ; and that eventually a 

 secondary segmentation of the yolk takes place around these 

 nuclei, and gives rise to a mass of yolk cells, which fill up the 

 interior of the embryo. These cells are diagrammatically shewn 

 in figs. 181 and 189, and it is probable that they constitute the 

 true hypoblast. Their further history is given below. 



Formation of the organs and their relation to the germinal 



layers. 



The segments and appendages. One of the earliest 

 phenomena in the development is the appearance of transverse 

 lines indicating segmentation (fig. 186). The transverse lines 

 are apparently caused by shallow superficial grooves, and also in 



1 Tichomiroff (No. 420) denies the existence of a true invagination to form the 

 mesoblast, and also asserts that a separation of mesoblast cells from the epiblast can 

 take place at other parts besides the median ventral line. 



