CTENOPHORA 



149 



I at its top, finally to spread out flat on the inner surface of the 

 ectoderm at the apex of the embryo. The plate thus formed, 

 which frees itself more and more from the entoderm, at first 

 elongates only in the direction of the transverse plane ; later, 

 however, by a new mesodermal growth from the original 

 centre, a cruciform mesodermal fundament (Fig. 71 m) is 

 formed, on which we can distinguish two longer (lateral) and 

 two shorter (sagittal) mesodermal bands. The former are 

 closely applied to the funda- 

 ments of the tentacles (Fig. 70 

 A and B), and supply the meso- 

 dermal axes, especially the 

 musculature, of the tentacles, 

 whereas the median (sagittal) 

 bands become the seat of the 



1^^ formation of migratory cells 

 ^B (Fig. 71 g), which wander into 

 ^^ the mesogloea and give rise to 

 the cellular elements of the 

 gelatinous tissue by becoming 

 metamorphosed there into stel- 

 late connective-tissue cells and 

 branched muscle fibres. 



In regard to the development of 

 the mesodermal structures we have 

 followed exclusively Metschnikoff's 

 (No. 16) description. Formerly the 

 origin of the elements of the gelatinous 

 tissue was attributed by Kowalevsky 

 (No. 14) and Chun (No. 3) to an im- 

 migration of ectodermal cells (the 

 superficial as well as the gastral epi- 

 thelium). According to Chun, this 

 immigration does not cease with the 

 embryonal stage, but throughout life 

 adds new muscle elements to the gela- 

 tinous tissue. The immigration of 

 ectodermal elements into the mesoglcea 

 during embryonic life is directly denied 



by Metschnikoff. Accordingly the gelatinous tissue would be essentially 

 a mesodermal formation; and even though in later stages ectodermal 



Fig. 70.— Two stages of the de- 

 velopment of Callianiva bialafa(after 

 Mktschnikoff, from Lang's Lehr- 

 huch). en, entodemi; me, meso- 

 derm ; me^, mesenchyma ; t, tentacle ; 

 sfc, sensory body ; d, intestinal 

 cavity; st, oesophagus (fundament 

 of the stomachj ; g, mesogloea. 



