Habtts and Early Development of Linergcs incri-iirius. 163 



through the increased size of the blastoccel (figs. 29 to 32, 47, 48). At the 

 same time the contents of the blastoccel become more fluid and stain less 

 deeply, as compared with the earlier stages. 



In a stage with about 1,000 cells (fig. 32), clear protoplasmic processes 

 which resemble pseudopodia appear over the entire periphery of the embryo, 

 save a small area at the vegetal pole. These processes are usually blunt- 

 conical in shape, though some of them are very irregular. They lie under 

 the egg-membrane, which they lift from the surface of the egg. These 

 processes are the first steps in the formation of the cilia which ultimately 

 clothe the entire outer surface of the larva. At first they move slowly and 

 irregularly, liut later, as they grow more slender, they vibrate in a typical 

 manner. 



Gastrulation usually takes place by invagination (figs. 33, 34, 47 to 49). 

 The small rounded cells at the vegetal pole are pushed into the blastoccel 

 and become the walls of the enteron, a flask-shaped cavity which opens to 

 the e-xterior through the blastopore at the vegetal pole. The blastopore 

 soon closes, so that the enteron is shut off completely from the exterior 

 (fiS''- 35i 36, 50). Sometimes gastrulation takes place by the immigration 

 of a mass of endoderm cells at the vegetal pole, and in such cases there is at 

 first no enteric cavity in this mass of entoderm cells. Later these cells 

 separate and arrange themselves around an enteric cavity, and the end 

 result is the same as in cases of typical invagination. The close relationship 

 between unipolar immigration and invagination is thus clearly shown by the 

 occurrence of both processes in different eggs of the same animal. 



In other genera of Scyphomedusx all forms of gastrulation (invagina- 

 tion, immigration, delamination) occur. This fact indicates that the form of 

 gastrulation is of no fimdamental or general significance, but that it depends 

 upon individual or environmental conditions. 



Planula. — After the closure of the blastopore tlie embryo elongates and 

 becomes a free-swimming planula (figs. 35, 36). The endoderm no longer 

 forms a simple layer, but consists of a more or less irregular mass of cells, 

 within which is the enteric cavity (figs. 35, 36, 50). In many cases (perhaps 

 in all) several small ingrowths of ectoderm cells into the space between 

 ectoderm and endoderm takes place (plate 8, fig. 50. te.xt-figs. i, 2). These 

 ectodermal masses then become hollow. Owing to my failure to obtain 

 material of the later stages of development I have been unable to determine 

 their significance. 



The latest stage in the development of Linerges which 1 liave seen, cor- 

 responds to plate 8, figure 50 and to text-figures i and 2. At this stage there 

 is no opening into the enteric cavity, though the ectodermal invagination 

 shown at the narrower end of the larva in text-figure 2 mav represent the 

 formation of the mouth. With this stage — the free-swimming planula — my 

 observations on the normal development come to an end. 



