90 EMBRYOLOGY 



menfc there always arises the same larval form, with identical 

 structure : a bilaminar, thickly ciliated, oval, pyriform or 

 more elongated vermiform planula, which possesses an 

 ectoderm composed of prismatic or columnar cells, an 

 entodermic epithelium consistino^ of large cubical elements, 

 and a homogeneous membrane (sustentative lamella), which 

 is secreted between the two layers at an early period. The 

 internal cavity of this larva (gastral cavity) is in most cases 

 still filled with masses of food-yolk. In this swarming stage 

 there can be recognized a broader, anterior, aboral end of the 

 body, which subsequently serves for attachment, and is fre- 

 quently characterized by a long tuft of cilia and a narrower 

 posterior end ; here the oesophagus is formed by invagina- 

 tion, and at its deepest part a communication with the 

 gastral cavity is produced by resorption of the cells. The 

 further development takes place principally by the formation 

 of the mesenterial septa, the filaments, the tentacles, and, 

 finally, in the Corallia (Madreporaria), the calcareous 

 skeleton. 



As regards the sequence in the development of the septa, 

 the views expressed by Milne-Edwards et Hatme, based 

 chiefly upon the condition of the tentacles and calcareous 

 septa of the adult animal, were formerly generally accepted. 

 According to them, first six primary septa are simultaneously 

 developed, then six of the second order in the interspaces 

 between these, then twelve septa of the third order, twenty- 

 four septa of the fourth order, and so on, the septa of each 

 newly appearing cycle being interpolated, as was maintained, 

 between those already present. On the other hand, we owe 

 to the investigations of Lacaze-Duthiers (No. 89) the know- 

 ledge that this regular arrangement, which is based on the 

 number 6, is a secondary one, and that the septa of a cycle 

 are formed at difl^erent times, becoming equalized only sub- 

 sequently. Most important of all in the earliest stages is a 

 well-marked bilaterally symmetrical condition, and the 

 stages with four and with eight septa are to a certain extent 

 well marked, whereas the intermediate stage, with six 

 primary septa, is a very transitory one. As regards details, 

 the statements of Lacaze-Duthiers on the sequence in the 



