10 



Ilybridizalion of Echinoids. 



formation of primary mesenchyme and in the form of the pluteus. 

 The only detailed description I have found of the early development 

 of a Cidarid is that of Prouho (1887) on Dorocidaris papillata. The 

 rate of development of this form is very slow, slower than that which 

 I have recorded for Cidaris irihuloides. The stage of the gastriila 

 with a primary enterocoele is reached in 6 days; the enterocoele has 

 divided to form two pouches in 8 days; while in 10 days skeletal 

 rods with four openings are present. Prouho kept larva? of this 

 species alive for 3 months and obtained plutei with 3 pairs of larval 

 arms and of the characteristic echinopluteus form. 



Fig. 4ffl, Free-hand sketch of Uving Cidaris larva in ventral view; 66 hours. 



46, Camera sketch of living Cidaris larva in ventral view; 73 hours. X 135. 

 4c, Camera sketch of living Cidaris larva in ventral view; 6 days. X 135. 



Prouho's description of the process of mesenchyme formation is 

 not complete enough to enable me to decide whether the process in 

 these two Cidarids is identical. He describes the mesenchyme as 

 budding into the blastocoele from the posterior wall of the blastula, 

 and gives a figure illustrating the process. This figure would serve 

 as a satisfactory illustration of beginning gastrulation, as I have 

 seen it, in Cidaris irihuloides. Without question the first cells that 

 are carried into the blastocoele in tribuloides, as invagination begins, 

 are the future mesenchyme cells, but these cells do not leave their 

 position in the wall of the archenteron until this has grown well 

 toward the center of the blastocoele. The appearance is somewhat 

 that of mass budding. It might be called such if the cells formed a 



