400 



EMBRYOLOGY 



\^ mesoderm, which constitute in the Annelida, as well as in some other 

 forms, the starting-point for the formation of the mesoderm (comp. pp. 

 264 and 282). 



Two groups of cells arise by the multiplication of the primitive mesen- 

 chyraa cells, either by their being forced into the blastula by the pressure 

 of the surrounding cells (Fig. 185, after Selenka), or by new cells being 

 constricted off from them (Flkischmank). Out of these groups there are 

 said finally to arise two bilaterally symmetrical cell-bands, which corre- 

 spond to the mesodermal bands of the Annelida. As a result of these 

 processes, the larva would show at a very early stage bilateral symmetry, 

 which becomes intensified by the flattening which takes place from the 

 dorsal to the ventral side. 



This conception of the origin of the mesenchyma from the two 

 primitive mesenchyma cells was generalized by Selenka, for he also 



found the two primitive cells 

 in other Echinoderms (Holo- 

 thiirioidea and Ophiuroidea). 

 Thus in Synapta there are the 

 two cells lying at the summit 

 of the archenteron (Fig. 175), 

 and also in Ophioglypha two 

 cells which detach themselves 

 from the cells of the blastula. 

 This kind of mesenchyma de- 

 velopment is not by any means 

 so typical as it is said by 

 Selenka and Hatschek to be 

 in the Echinoidea, and recalls 

 much more the formation at 

 pleasure of mesenchyma cells 

 which, like the succeeding ones, 

 separate from the cell-wall of 

 the blastula or gastrula, as is 

 known to take place in the 

 Agteroidea and Crinoidea (comp. pp. 402 and 403). Metschnikoff has 

 accordingly opposed the theory of mesenchyma formation espoused by 

 Selenka and Hatschek, and, on the whole, we agree with his con- 

 clusions. 



The appearances which led Hatschek and Selenka to believe in the 

 existence of primitive mesenchyma cells are to be explained by the fact 

 that the cells of the blastula at the time of dividing are shortened, and 

 become stouter. Thus it happens that directly after the division of such 

 a shortened cell two small cells, surrounded by tall, prismatic cells, come 

 to lie side by side, and thus such stages arise as those of Hatschek and 

 Selenka (Figs. 183 and 184). Such shortened cells occur in various 

 parts of tlie circumference of the blastula, when the development of the 



Fig. 185. — Blastula stage of Strong]ilo- 

 centrolus lioidus, with mesenchyma cells 

 which have migrated into the blastocoele 

 (after Sklbnka) 



