Habits and Early Dei'clopnicnt of Liucvfjcs iiirrniriiis. 163 



through the increased size of the blastocoel (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 psendojiodia 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, but 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 blastocoel 

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

 the exterior through the blastopore at the vegetal pole. The blastopore 

 soon closes, so that the enteron is shut ofif completely from the e.xterior 

 Cfigs. 35, 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 inmiigration and invagination is thus clearly shown by the 

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



In other genera of Scyphomedus?e all forms of gastrulation ( invagina- 

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

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

 upon individual or environmental conditions. 



Planula. After the closure of the blastopore the 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, text-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 Liiicrges which I have 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 may represent the 

 formation of the mouth. \\'ith this stage the free-swimming planula my 

 observations on the normal development come to an end. 



