170 EMBRYOLOGY. 



CHAPTER IX. 



DEVELOPMENT OF CONNECTIVE SUBSTANCE AND BLOOD. 

 (THE MESENCHYME-THEORY.) 



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

 tentative substance and blood we enter a very difficult field, the 

 cultivation of which has now been taken in tand 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 

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



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

 from BALFOUR. 



A, Blastosphere-stage at the end of cleavage. 



B, Gastrula-stage. 



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

 secreted as a gelatinous core ; bl, blastoderm ; ep, outer, hy, inner germ-layer ; ms, 

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



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

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

 very early ; in the Echinoderms, for example, as early as the blastula- 

 stage (fig. 109). 



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

 jelly-core (s.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 



