260 Elementary Biology. 



in the ccelomic space, which is continued down into the 

 outer walls of the atrial cavity. There are no reproductive 

 ducts nor openings. The ova and sperms are shed by the 

 rupture of the inner wall of the atrial fold into the atrial 

 cavity, the former probably escaping by the buccal cavity to 

 the exterior after passing through the branchial slits, the 

 latter probably by the atrial pore. 



Fertilisation and development. The ova are fertilised 

 by the sperms after leaving the body of the female. The 

 ovum is enclosed in a delicate cell-wall. The embryo, or 

 fertilised ovum, immediately segments, and by successive 

 division the originally unicellular ovum becomes a multi- 

 cellular hollow sac or ball termed a blast o sphere. When 

 fully formed one side of the blastosphere becomes invagi- 

 nated, or pushed inwards, so that a double-walled cup is 

 formed, the shell membrane being meantime cast off. The 

 cup elongates into a flask, and the originally wide open mouth, 

 where the inner and outer walls were continuous, becomes 

 narrowed to a small aperture the blastopore. While in 

 this condition the outer wall of the embryo is ciliated, and 

 the body rotates freely in the water. The embryo is now a 

 gastrula, composed externally of a layer of ciliated ecto- 

 derm, and internally of long columnar endoderm cells. 



At this point there follow some very important changes. 

 First, the embryo becomes flatter on one side. This 

 flattened area, known as the medullary plate, becomes 

 converted into a groove or valley, partly by the formation of 

 two ridges, which rise up along the sides of the plate, and 

 partly by the downward curving of the centre of the plate 

 itself. The folds, or dorsal laminae, gradually enclose the 

 blastopore, and unite together behind it. The blastopore 

 is thus made to open on what will become the dorsal sur- 

 face. The laminae unite above the medullary groove from 

 behind (the blastopore end) forwards, roofing over a tubular 

 cavity formed partly by the laminae as a roof, and partly 

 by the curved medullary plate as sides and floor. This 



