170 EMBRYOLOGY. 



CHAPTER IX. 



DEVELOPMENT OF CONNECTIVE SUBSTANCE AND BLOOD. 

 (THE MESENCHYME-THEORY.) 



WITH the question of the origin of connective or mechanically sus- 

 t-iitativt; .substance and blood we enter a very difficult field, the 

 cult ivation of which has now been taken in hand successfully by many 

 persons. Here also we shall acquaint ourselves with a simple case 

 from the development of Invertebrates, before we begin with the 

 conditions in Vertebrates, which are more difficult to comprehend. 



In Coelenterates and Echinoderms there is developed between the 

 germ-layers, which are composed of epithelial cells, a sustentative 

 tiue. It consists of a homogeneous jelly, in which are scattered a 



Fig. 109. -Two stages of development of Holothuria tubulosa, in optical section (after SEUESKA), 

 from liALFiiru. 



A, Blastospbere-stage at the end of cleavage. 



B, (j.-utni la-stage. 



mr, Micropyle ; Jl, chorion ; .c, segmentation-cavity, in which gelatinous substance is early 

 secreted as a gelatinous core ; M, blastoderm ; ep, outer, Ay, inner germ-layer ; nu, 

 amoeboid cells arising from the inner germ-layer ; a.e, coelenteron (archenteron). 



few isolated spheroidal or stellate cells, which are capable of changing 

 jMMtinn by virtue of their amoeboid motion. It is usually developed 

 \TV parly ; in the Echinoderms, for example, as early as the blastula- 

 stage (fig. 109). 



Into the cavity of the blastula (A) a homogeneous soft sul stance, the 

 jplly-core (*.c), is secreted by the epithelial cells. Into this jelly there 

 migrate from the epithelium, and indeed from the particular region 

 which at the time of gastrulation is infolded (fig. 109 B) as the 



