The first stage corresponds to the process of organization, the gelatine representing the blastema, and the drop the nucleus.
"The Mechanism of Life"
Stéphane Leduc
An involution of the integument takes place on one side of the back, and continues to deepen till it reaches a mass or store of what is called blastema, or the raw material of the animal body.
"On the Origin and Metamorphoses of Insects"
Sir John Lubbock
The structures which presently arise round nuclei in a blastema, and which have in some way been determined by those nuclei as centres of influence, evidently conform to the law; for the parts of the blastema in contact with the nuclei are differently conditioned from the parts not in contact with them.
"Essays: Scientific, Political, & Speculative, Vol. I"
Herbert Spencer