CHAPTER IV. 

 EARLY DEVELOPMENT OF AMPfflOXUS. 



Although the ova of Amphioxus are not used extensively for teaching 

 purposes in the laboratory, a study of the early developmental stages is a 

 valuable aid to the reasonable comprehension of certain embryological facts. 

 The simplicity of these first steps, whether it points to primitiveness or not, 

 affords a view of certain fundamental principles of development which makes 

 the study of higher vertebrate forms much easier and renders their formative 

 processes much more intelligible. This simplicity is probably correlated with 

 the freedom of the egg from a large amount of yolk; and it will be seen that 

 many of the modifications of the processes of development in the vertebrates 

 seem to be produced by the greater amount of yolk in their ova. 



Cleavage. The ovum of Amphioxus has certain peculiarities which are 

 important in their effect upon cleavage. While it contains only a small 





PV 



FIG. 1 8. Diagram of a median sagittal section through an Amphioxus ovum. Cerfontaine, 



from Kellicott. 

 The arrow indicates the direction of the polar axis. AD, antero-dorsal region; PV, postero- 



ventral region; N, male and female pronuclei; p, yolk-free area; S, tail of sperm; y, yolk 



area; II, second polar body. 



quantity of yolk, being regarded as a meiolecithal ovum, this material is 

 situated slightly off center and the nucleus lies outside of the yolk (Fig. 18). 

 This condition really effects a polarity of the cell. The first polar body is 

 given off from the yolk-free portion of the egg. This marks the animal pole 

 and also the side which will be the anterior part of the embryo. The sperm 

 enters the egg at the vegetative pole and seems to stimulate the formation of 



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