CHAPTER 14 



Blastula, Gastiula and Pluteus 



a. Blastula and Gastrula. Plates III and IV 



By counting the number of cells at the periphery of an optical section 

 of a Hving blastula, it has been calculated that there are approx- 

 imately 1,000 cells in an Arbacia blastula just before hatching; this 

 would represent approximately 2'° or ten cleavages. MacBride (1914) 

 estimated 1,000 for Psammechinus microtuberculatus and Morgan (1895 c) 

 500-525 for Sphaer echinus granulans, about nine divisions. Soon each 

 cell acquires a cilium and rotates inside the fertilization membrane. 

 About 8 hours after fertilization at 23° C. (7I to 9^ hours in different 

 batches) it breaks through the fertilization membrane and becomes 

 free-swimming. This is done by means of a "hatching enzyme" which 

 dissolves the membrane. Such an enzyme was found many years ago 

 in fish eggs, in Lepidosiren by Kerr in 1900, and was especially studied 

 in the Ascidians by Berrill (1929). In sea urchins its occurrence was 

 reported by Ishida (1936) in Strongylocentrotus pulcherrimus, and later 

 by Kopac (1941) in Arbacia punctulata. The Japanese species was studied 

 again more recently by Sugawara (1943 a). 



The blastocoel of the Arbacia tgg is usually small on hatching, in 

 contrast to many other sea urchins, e.g., Strongylocentrotus drobachiensis, 

 Lytechinus variegatus. In Arbacia the blastocoel gradually becomes larger 

 leaving only a thin layer of ciliated cells at the periphery of the blastula. 

 The cilia are longer at the apical pole as early as hatching, forming the 

 apical tuft. 



After the blastula has become free-swimming, it remains spherical 

 for about seven hours, swimming very actively. Then it begins to 

 invaginate (about 15 hours after fertilization) at the pole where the 

 micromeres came off, the original vegetal pole, and opposite the pole 

 where the polar bodies were given off, the animal pole (Morgan and 

 Spooner, 1909; Horstadius, 1937a). This relationship was beautifully 

 shown for the Paracentrotus lividus egg in the classic studies of Boveri 

 (190 1, etc.) and is the same for Arbacia. 



The invagination continues until it approaches the anterior end 



