61 



Patten has been able to follow the early stages of 

 development. 



Tlie fertilized ovum segments into two, and llien into 

 four fairly ecjual cells. Subsequent divisions arc 



markedly unequal, giving rise to numerous small cells 

 (future ectoderm), and, less rapidly, tf) a few larger cells 

 (future endoderm and mesoderm). The " Gastrula " 

 stage is initiated by growth of the snialk^r (ectoderm) cells 

 over the large cells, i.e., by epiboly. The l)lastopore is 

 the spot over which the small cells do not spread. By 

 differential growth a blastoccele is now formed, into which 

 the inner ends of the large cells grow. These cells bud 

 off smaller ones towards the blastocoele, and from one of 

 these are developed the two primary mesoderm cells. 



The ectoderm is mostly ciliated at first, but two adjacent 

 transverse rows of cells soon develop larger and more 

 conspicuous cilia, thus forming two (pre-blastoporal) 

 ciliated rings around the embryo. These together form 

 the " Prototroch," the first rudiment of the " A'elum." 

 The apical cells of the embryo also develop very long 

 cilia, and those around them shorter but still conspicuous 

 ones. These ciliated cells lengthen and sink, forming a 

 fairly flat apical plate. At the opposite pole of the 

 embryo two ectoderm cells increase in size and also bear 

 cilia. These are called the anal ciliated cells. Another 

 group of ectoderm cells, posterior to the Prototroch and 

 ou the dorsal side, become depressed and lengthened. 

 These form the shell gland which increases in extent 

 especially at the back, making this part of the surface 

 of the embryo convex, and shifting the ventrally-placed 

 blastopore relatively forAvards. Meanwhile the blastopore 

 changes its relative position, becoming U-shaped and then 

 slit-like. In the position of its most anterior portion, 

 there occurs an insinkiug of ectoderm, which pushes this 



