THE DEVELOPMENT OF STATOBLASTS. 51 



"bilaminar, consisting of an inner core of ectoderm and an outer layer 

 of mesoderm. At the lower end of this cone the first statoblast- 

 rudiments form, a group of ectoderm-cells (eg) becoming detached 

 from the central core and arranged round a small cavity, thus 

 forming a vesicle. This part of the statoblast-rudiment has been 

 named by ISTitsche the "cystigen half," as from it are formed the 

 cysts of the statoblasts. A second part of the rudiment, the so- 

 called "formative mass" (bm), arises from the outer mesodermal 

 layer of the funiculus. It represents the rudiment of the inner 

 mesodermal mass which, filled with particles of food-yolk, is found 

 in the statoblasts, while the "cystigen half" yields not only the 

 cell-layer which secretes the cyst but, as Reinhard has proved, the 

 ectodermal layer of the statoblast-germ (Fig. 26 A, a and b). 



According to Davenport (No. 46a) the fiinicuhis is to be traced back not so 

 much to a fold as to an active independent wandering of its component 

 mesoderm-cells. Kraepelin (No. 50) has stated that the inner layer of the 

 funiculus does not grow in from the zooecial wall, but from the internal lining 

 ■of the stomach-caecum at tlie opposite end of the strand. If we follow the 

 statements by Braem given above, the essential agreement between the rudi- 

 ment of the funiculus and tliat of a polypide-bud is very clear, so that we are 

 justified in regarding the statoblasts as internal buds. A view formerly held 

 by Verworn (No. 57) derived the cystigen half and the formative mass from a 

 single cell, which underwent a process of cleavage. Yerworn was therefore 

 inclined to regard the statoblasts as parthenogenetic winter eggs. 



As the statoblasts develop further, the complete circumcrescence of 

 the "formative mass" (Fig. 26, b) by the vesicular "cystigen half" 

 (a) takes place. The point last affected by this circumcrescence (jj), 

 where for some time an aperture can be seen, corresponds to the 

 middle point of the lower surface of the lens-shaped statoblast, 

 which is usually somewhat more convex than the other. The 

 ^'formative mass" is in this way enveloped by two layers of the 

 "cystigen half" (a' and a"). The inner layer (a") corresponds to 

 the ectoilerm of the statoblast, while the outer layer (a') is concerned 

 in the formation of the shell (r). The cells of this outer layer first 

 •secrete on their inner ends, i.e., on the side next to the ectoderm 

 of the statoblast, a cuticular cyst (c) which surrounds the statoblast. 

 In the substance of this cyst, a slit or line of demarcation soon 

 ■appears, corresponding to the equator of the cyst; this indicates 

 the division of the cyst into the two watch-glass-like halves which 

 .separate later to allow of the escape of the young colony. After 

 this chitinous envelope has been secreted, the marginal cells of the 

 .secreting layer elongate and form a fold round the statoblast. These 



