Page:Proceedings of the Royal Society of London Vol 60.djvu/59

Rh rounded by a single layer of derm al cells. Some of th e cells of th e dermal epithelium th en form them selves into groups, usually of three cells, and each cell of such a group secretes the ray of a spicule. The first spicules are usually triradiate, b u t quite irreg u lar in form, and at th eir first appearance th ey are quite superficial, th e ir secreting cells form ing p art of th e general epithelium, b u t later they become covered by th e rem aining epithelium, so th a t th e derm al layer becomes divided into an internal connective tissue layer and an external flat epithelium. The process is essentially sim ilar to th a t occurring in varibls, except th a t in th e la tte r the cells of the flat epithelium secrete each a m onaxon spicule, w hich in cerebrum is not the case.

The larva of L. variabilis is of in terest as affording a transition from larves such as th a t of L. reticu, to th e am phiblastula larva of th e Sycons. The larva of reticulum (fig. 7) is composed of (1) ciliated cells, com parable to those of the am phiblastula, of which some (2) at the hinder pole are undergoing modification, and m ay be compared w ith th e interm ediate cells, and of (3) internal granular cells comparable to the posterior g ranular cells of th e am phiblastula. To. obtain a larva like th a t of variabilis from th e type represented by reticulum, we m ust suppose the large cavity of the latter reduced to the extent to w hich th is has occurred in the former. Then the granular cells which are formed a t the posterior pole m ust rem ain where they are, since the cavity is too small to contain them, and, as more ciliated cells are continually being modified around them , we get a larva w ith the three kinds of cells arranged as in variabilis. The central cells of variabilis—on th e origin of w hich I have no observations to bring forw ard—are probably to be regarded as constituting a larval organ, a special adaptation of no im portance for the postlarval development.

The development of both reticulum and variabilis points to an early stage in w hich the larva is composed entirely of sim ilar and equivalent ciliated cells. I have not seen such a stage in any species, and doubt if it actually occurs in n a tu re ; it is more probable th at the process of cell differentiation begins before the larva is hatched in all cases. In the absence of segm entation stages, it is impossible to decide this question; nevertheless, the facts seem to me to indicate, as the prim itive larva in ascon phylogeny, a blastula composed of indifferent ciliated cells, in which a second type of cells (the future dermal layer) is formed by modification of certain of the cells. The collarcell layer of the adult is derived directly from the prim itive ciliated cells of the blastula.

Comparing, now, the larva of variabilis w ith that of Sycon raphanus,