348 



EMBRYOLOGY. 



In Amphioxus, the Cyclostomes, and the Amphibia the most 

 important function of the primitive segments is the production of the 

 fundament of the transversely striped and voluntary musculature. 

 On the other hand it is not very evident that the primitive segments 

 also share, in the manner previously (p. 172) described, in the deve- 

 lopment of the mesenchyme; 

 this is correlated with the fact 

 that in general the connective 

 and sustentative substances 

 play a slight role in the con- 

 struction of the bodies of the 

 lower Vertebrates, and es- 

 pecially during larval life are 

 developed to only a very insig- 

 nificant amount. 



This is altered in the Sela- 

 chians and the three higher 

 classes of Vertebrates. Not 

 only does the mesenchyme 

 in the adult bodies of these 

 attain a more voluminous 

 development and a degree of 

 differentiation that is in 

 all directions more advanced, 

 but it is also established 

 earlier and likewise in greater 

 abundance. Therefore the 

 primitive segments here ex- 

 hibit in their metamorphosis 

 somewhat modified pheno- 

 mena. At the same time 

 with the differentiation of 

 the muscular tissue, and in 

 part even before that event, 

 the development of mesen- 

 chyme is observable. The primitive segment (fig. 194) in this case 

 is differentiated from the start into two equally distinct fundaments, 

 of which the one is designated as sclerotome or skeletogenous layer 

 (sk), the other as muscle-plate (nip). While referring the reader to 

 the ninth chapter, I add to the presentation given there a few 

 further statements. 



Fig. 194.- Cross section through the region of the 

 pronephros of a Selachian embryo, in which 

 the muscle-segments [myotomes] (rnp) are in 

 process of being constricted off. Diagram 

 after WIJHE. 



nr, Neural tube ; ch, chorda ; ao, aorta ; sch, sub- 

 notochordal rod ; mp, muscle-plate of the 

 primitive segment ; w, zone of growth, where 

 the muscle-plate bands around into the cutis- 

 plate (cp) ; vb, tract connecting the primitive 

 segment with the body-cavity, out of which 

 are developed, among other things, the meso- 

 nephric tubules (fig. 205 &) ; sk, skeleto- 

 genous tissue, which arises by a proliferation 

 from the median wall of the connecting tract 

 vb ; vn, pronephros ; mk l , mk*, parietal and 

 visceral middle layer, from whose walls 

 mesenchyme is developed ; Ih, body-cavity ; 

 ik, entoblast. 



