760 DR. E. B. WILSON ON THE DEVELOPMENT OF RENILLA. 
II, 
DEVELOPMENT OF ORGANS AND TISSUES.° 
The larva now consists of a layer of ectoderm and entoderm separated by the 
supporting lamella and enclosing the gastric cavity. The latter has as yet no 
communication with the exterior and shows no trace of division into the eight 
radiating chambers characteristic of all Aleyonarian polyps. Within a few hours— 
usually between the fortieth and fiftieth—the cesophagus is formed, though it is not 
perforated until a far later period, and the gastric cavity is divided into chambers 
through the appearance of radiating septa. These structures develop simultaneously, 
but it will be convenient to follow their formation separately, and the same plan will 
be followed in describing the development of other organs. 
§ 8. Formation of the esophagus and mouth. 
The esophagus usually makes its appearance in the larva of about forty hours as a 
solid invagination of ectoderm at the larger end of the body (fig. 134). The high 
columnar ectoderm cells at this point change their form entirely and rapidly multiply, 
and are pushed into the body of the larva as a solid plug (fig. 134, s¢.). The 
invaginated cells are very small, rounded, possess distinct but very small nuclei, and 
are so closely packed together that their outlines can scarcely be distinguished except 
in very thin sections. They differ widely from the entoderm cells, being far smaller, 
staining more deeply and of a different tint, and their nuclei are much smaller. In 
some cases the cesophagus contains from the first a very small cavity extending 
inwards from the exterior, but it is usually quite solid. As the plug of ectoderm is 
pushed in, it carries before it the supporting lamella and the entoderm, the cells of 
the latter multiplying at the same time. These entodermic cells assume a high 
columnar form, with their long axes directed towards the cesophagus (fig. 134). 
The ectodermie plug grows rapidly backwards and assumes a somewhat pyriform 
shape from the expansion of its lower extremity (fig. 136), and the entoderm, which 
everywhere covers it, often becomes much thickened, especially at its lower end. A 
narrow cavity (fig. 135) then appears in its centre, communicating at the anterior 
extremity with the exterior, but still ending blindly below. The cavity appears to be 
formed by the giving way of the central cells, aided to some extent, perhaps, by 
absorption. In this stage the cells towards the outer opening sometimes have an 
obseurely columnar form, but towards the inner end of the invagination are small and 
rounded as before. The cavity at its very first appearance, as shown in transverse 
sections, is greatly elongated in a particular direction, which is always the same in 
relation to the septa and is shown by the later development to coincide with the 
dorso-ventral axis. 
