61 



Patten lias been able to follow tlie early stages of 

 development. 



The fertilized ovimi segments into two, and then into 

 four fairly equal cells. Subsequent divisions are 

 markedly unequal, giving- rise to numerous small cells 

 (future ectoderm), and, less lajndly, to a few larger cells 

 (future endoderm and mesoderm). The " Gastrula " 

 stage is initiated by growth of the smaller (ectoderm) cells 

 over the large cells, i.e., by epiboly. i'lie blastopore is 

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

 dii¥erential growth a blastocoele is now formed, into which 

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

 off smaller ones towards the blastocfele, 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 hrst rudiment of the " Velum." 

 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 tlat apical plate. At the opposite pole of the 

 embryo two ectoderm cells increase in size and also bear 

 cilia. These are called tiie anal ciliated cells. Another 

 group of ectoderm cells, posterior to the Prototroch and 

 on 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 forwards. Meanwhile the blastopore 

 changes its relative position, becoming U-shaped and then 

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

 tliere occurs an insiuking of ectoderm, which pushes this 



