COMPARATIVE EMBRYOLOGY 807 



illustration is seen in the scorpions, which might be considered as an 

 extreme case of superficial cleavage. 



Superficial cleavage (Fig. 414) predominates in the arthropods, al- 

 though not all arthropods have this type of cleavage. It occurs also in 

 some coelenterates, and it has been described for one sea cucumber. In 

 superficial cleavage the yolk is in the form of a great many spheroids 

 which are separated from each other by very thin cytoplasmic films. 

 In the center of the uncleaved egg the nucleus lies in an island of 

 cytoplasm. Cleavage is accomplished by the repeated divisions of 

 the nucleus and the surrounding cytoplasm. These small nuclear 

 cytoplasmic islands gradually move about, and as their division con- 

 tinues, they are arranged into the form of a layer of cells which finally 

 reaches the surface of the egg and forms the blastoderm there. This 

 is a superficial layer with the yolk mass left in the center of the egg 

 and constitutes the so-called superficial blastula. Some cells remain 

 behind to help digest the yolk and are known as vitellophages. 



The cleavage period, regardless of the type, terminates with the 

 formation of the blastula, a one-celled embryo. Of the blastulae there 

 are seven kinds. Most students of embryology think of cleavage in 

 terms of the holoblastic eggs, and of blastulae as hollow spheres of 

 cells of the type of the coelohlastula. Actually this is only one of the 

 varied kinds of structures, all of which answer the definition of a 

 blastula, "a one-layered embryo." 



Eggs having radial cleavage usually develop into coeloblastulae, 

 as do those other holoblastic types in which the amount of yolk is 

 smaU. Coeloblastulae, that is, blastulae containing cleavage cavities, 

 may be either equal or unequal, depending upon the amount of yolk 

 present. If the size of the cavity is greatly reduced, we approach 

 the second type, the stereohlastula, which is a solid embryo, but here 

 again the dimensions of the cells may be uniform or quite variable, 

 and we likewise have equal and unequal stereoblastulae. In the 

 stereohlastula, however, all of the cells reach the surface of the entire 

 embryo, none of them being cut oK at the interior as in the case of the 

 type of blastula known as the morula. Formerly, it was common to 

 speak of the morula as a stage in development of any embryo, but this 

 usage has been abandoned and the term is now given to a particular 

 type of solid blastula in which certain cells are entirely cut off from 

 the surface of the embryo. The placula is a type of blastula in which 

 the cavity is reduced by a shortening of the egg axis which runs from 



