This embryo, resembling a hollow rubber ball filled with fluid, is called a blastosphere.
"The Whence and the Whither of Man" by John Mason Tyler
The segmentation is complete; one side of the hollow blastosphere invaginates and forms a gastrula.
"Encyclopaedia Britannica, 11th Edition, Volume 5, Slice 7" by Various
The central part of the blastosphere is called the segmentation cavity or blastocoel.
"Encyclopaedia Britannica, 11th Edition, Volume 9, Slice 3" by Various
Thus, an oval and an elongate egg produce respectively an oval and an elongate blastosphere.
"The Biological Problem of To-day" by Oscar Hertwig