50 



MESODERMIC SEGMENTS 



transversely across the mass, of a process of thinning into a linear series of 

 small cubical masses (fig. 74), the mesodermic or primitive segments. 1 The 

 first pair of these segments appears a short distance in front of Hensen's 

 knot (fig. 75), in what will ultimately become the junction of the head and 

 trunk of the embryo. They are produced in succession from before backwards, 

 being gradually added as the embryonal axis increases in length, until the full 

 number (thirty-five or more for the human embryo) is laid down. It has been 

 shown in lower forms that the earliest segment to appear is not the most anterior 



IV 



- 



" '.-.-* 



FIG. 73. A SEBIES or TBANSVEBSE SECTIONS THBOUGH AN EMBBYO OF THE DOG. (After Bonnet.) 

 Section I. is the most anterior. In V. the neural plate is spread out nearly flat. 



The series shows the uprising of the neural folds to form the neural canal. 

 ect, ectoderm; ent, entoderm; mes, mesoderm; so, segment ; c, intermediate cell-mass ; l.p., lateral 

 plate still undivided in I., II., and III. : in IV. and V. split into somatopleuric (sm) and splanchno- 

 pleuric (sp) lamellae ; p, pericardium ; h, h, rudiments of endothelial heart-tubes. In III., IV.. and V. the 

 scattered cells represented between the entoderm and splanchnic layer of mesoderm are the vaso- 

 formative cells which give origin in front, according to Bonnet, to the heart-tubes (h) ; (a) aortae. 



of the series, as a number of head-segments develop from behind forwards in front 

 of that first differentiating. In the human embryo there are probably three such. 

 The most anterior segment, in higher vertebrates, lies some distance behind the 

 head end of the notochord in the future occipital region, and there is no trace of 

 segmentation in front of this point. 2 As the segments are being cut out of the 



1 Formerly known as ' protovertebrae.' The term 'somite ' is also frequently employed to designate 

 them. 



2 In Petromyzon and Selachians the mesoderm is segmented at least as far forwards as the 

 notochord extends ; the segments in front of the occipital region undergo retrogressive changes, and 

 disappear at an early stage. 



