164 Papers from the Marine Biological Laboratory at Tortngas. 



FlG3. I AND 2. Longitudinal sections of the advanced planulae, showing the columnar 

 ectoderm, the more or less solid endoderm, containing irregular or radiating cavities, 

 and the ectodermal invaginations which lie between the ectoderm and endoderm and 

 are of doubtful significance. An ectodermal invagination at the narrower pole of 

 fig. 2 may represent the stomodaeal invagination. 



As compared with the development of other medusje, the entire enihry- 

 ology of Liiiergcs is characterized by the regularity of the processes of 

 cleavage and gastrulation ; and although this regularity may sutler certain 

 modifications, without preventing the formation of a normal planula, there 

 is in this species none of that extreme irregularity which characterizes the 

 development of Pcnnaria ( Hargitt. 1904). 



EXPERIMENTS. 



Isolation of blastomercs. My observations on the development of parts 

 01 the unsegmented egg and of isolated blastomeres are essentially similar 

 to those of Zoja (1895) and Maas (1905). Parts of the unsegmented but 

 fertilized egg may give rise to swimming larvae; these are almost certainly 

 the parts containing the egg and sperm nuclei. Isolated blastomeres, at 

 least as late as the 4-cell stage, give rise to swimming larvae, which are ap- 

 parently normal; however the lack of clearly dififeraitiated organs in the 

 planula makes it difficult to determine in this stage whether the larvae are 

 wholly normal or not. When the egg fragments are small, or when the 

 blastomeres are isolated at a late stage of the cleavage, the blastocoel is rela- 

 tively small and the gastrulation is not normal. These results are essentially 

 like those obtained by all investigators of the development of the Cnidaria. 



