PART II 

 EMBRYOLOGICAL TYPES 



CHAPTER XI 



AMPHIOXUS 



Fertilisation. — The egg is surrounded by a vitelline membrane 

 secreted by itself, and contains yolk mostly aggregated at one 

 (the vegetative) pole. It is freed from the ovary and makes 

 its way to the outside via the atrium and atriopore, at a stage 

 shortly after the extrusion of the first polar body. In the water 

 a sperm penetrates into the egg, which then proceeds to give 

 off the second polar body ; the egg and sperm pronuclei then 

 fuse and fertilisation is effected. The second polar body 

 marks the animal pole of the egg, and it persists throughout 

 cleavage until the beginning of gastrulation, when it is possible 

 to see that the future anterior end of the embryo arises at a 

 point near the animal pole. Actually the axis of the egg (from 

 animal to vegetative pole) makes an angle of 30 with the 

 antero-posterior axis of the embryo. The egg-axis is deter- 

 mined in the ovary by the position of attachment of the egg to 

 the germinal epithelium. The dorso-ventral median plane of 

 symmetry of the embryo is marked by the point of entrance 

 of the sperm. 



Cleavage. — The cleavage of the egg is total or holoblastic, 

 i.e. the amount of yolk present is insufficient to prevent cell- 

 division, but the cells of the vegetative pole are larger than 

 those at the animal pole. Up to the 8-cell stage, the cell 

 divisions keep pace with one another, but after that they 



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