TYPES OF CLEAVAGE 337 



cleavage takes place the yolk does not divide and the cleavage 

 only extends through the protoplasm, which is thus divided 

 into a number of minute hummocks. From these, cells are 

 later cut off by horizontal cleavage planes, and the same process 

 extends outward and downward, continuously adding to the 

 number of fully formed cells. This type of cleavage is called 

 partial and discoidal, the latter referring to the disk-hke form 

 of the segmenting area. For most Arthropods cleavage follows 

 still another course. Here the eggs are also laden with yolk, 

 which is concentrated at the centre of the egg, while the proto- 

 plasm lies more equally distributed over the entire surface. 

 When the nucleus divides there is at first no division of the 

 cytoplasm. But after a time the nuclei arrange themselves 

 near the surface in a single layer. The cytoplasm then divides 

 partially as in the preceding case, but over the whole surface 

 of the egg. This type is distinguished as partial and superficial 

 cleavage. 



719. Origin of the Tissues.^If allowance is made for the 

 modification caused by the yolk, one may say that during the 

 early stages of development all metazoa proceed along parallel 

 lines. In all cases a blastula is formed, and this is followed by 

 a gastrula stage. The formation of a distinct mesoderm, how- 

 ever, does not occur in the Porifera and Coelenterates. These 

 animals, even in the adult, consist only of two distinct cell 

 layers, the ectoderm and entoderm, though there is usually a 

 supporting layer between. This may be simply a structureless 

 membrane or, in some cases, a thick layer composed chiefly of 

 a gelatinous matrix. Some of the ectodermal and entodermal 

 cells may send nervous or muscular fibre processes into this 

 layer, and there may also be few or many cells of ectodermal or 

 entodermal origin completely embedded in this layer. In the 

 other phyla, where a true mesoderm is formed, there is consider- 

 able variation in the method. It is almost always wholly ento- 

 dermal in origin, but some times it begins as a solid outgrowth, 

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