484 INVEETEBEATA chap. 



blastula of Asterias does not develop mesenchyme cells, these are 

 only formed after the gastrula stage has been reached. In these 

 abnormal blastulae the mesenchyme cells are given off from the 

 vegetative pole, while from this same pole mesenchyme is given off 

 in the normal blastula of Echinus before an archenteron has been 

 formed. 



Herbst concludes that the action of the sulphocyanide has 

 changed the development of the blastula from the Asteroid to the 

 Echinoid type. This may be admitted, but the question arises how 

 this comes about. The vegetative pole of the egg is the region which, 

 in the Asteroid, is invaginated to form the archenteron, and from 

 the apex of the archenteron the mesenchyme is given off; so that 

 the mesenchyme originates from the same region in the normal and 

 in the abnormal larva, Further, there is often, as we have seen in 

 the abnormal larva, an attempt to form an archenteron which is, 

 however, abortive. 



We may draw the conclusion, therefore, that the normal develop- 

 ment consists of distinct processes which are to a large extent 

 independent of one another. On these processes the drug acts in a 

 selective way ; it inhibits the process of forming an archenteron, but 

 it permits the formation of mesenchyme. We might go on to 

 suggest that the early development of both Asteroidea and Echinoidea 

 is made up of the same processes, but that in the case of the 

 Echinoidea the mesenchyme development is hurried on. In this 

 case it is not impossible that the hurrying on of the mesenchyme 

 formation in the blastula of Echinoidea is due to the formation of 

 some substance which acts in a similar manner to the sulphocyanide, 

 and that, therefore, one main difference between the eggs of 

 Asteroidea and Echinoidea is the presence of this substance in the 

 latter type of eggs, and its absence in the former. 



OPHIUEOIDBA 



Leaving now the Asteroidea, let us turn to the consideration of 

 the group most nearly allied to them, the Ophiuroidea. The develop- 

 ment of three species of Ophiuroids is known, viz, Ophiothrix fragilis, 

 Ophiura brevispina, and Amphiura squamata. Of these only the 

 first possesses a long larval development comparable with that of 

 Asterias, and we shall therefore select it as type for special descrip- 

 tion. Its development has been worked out by us (1907). 



Ophiura hrevispina has a yolky egg and a much modified larva, 

 which takes no nourishment and creeps slowly about on the bottom, 

 and therefore exhibits a development in some respects parallel to 

 that of Cribrella and Solaster. ' Amphiura squamata has a develop- 

 ment which is passed entirely within a pocket in the maternal body 

 — the genital bursa — which acts as a womb, from which the 

 embryo emerges as a star-fish similar to the adult in all important 

 respects. 



