﻿Contr. toward the Embryol. and Anat. of Pol. pallipes (Hymenopt.). 125 



rounded and then flattened against each other; they finally were cut 

 off from the yolk and formed a continous layer over the egg (Blasto- 

 derin). The cells which formed this layer originate by a process of 

 budding. He describes a second and a third layer, both also formed 

 by budding. 



Weismann (59), working in 1865 on the development of the 

 Diptera, noticed the clear peripheral layer covering the egg, which 

 had been seen by others, and called it the Keimhautblastem. In this 

 the blastoderm cells appeared by a process of free cell formation 

 and spread out upon the eggs surface. The nuclei appear first, they 

 are cut off in little masses of protoplasm to form the cells, which, 

 rounded at first, later become prismatic. An inner Keimhautblastem 

 appears just inside of the blastoderm which it supplies with nourish- 

 ment, gradually decreasing in amount and finally disappearing. In 

 Musca Weismann describes the Keimhautblastem as appearing first 

 at the anterior pole of the egg and from here spreading over the 

 entire surface. He also notices that cells of the blastoderm inigrate 

 into the yolk and noted the division of the blastoderm cells. 

 Kupffer (34), working in 1866 on the development of Chironomus, 

 agrees with Weismann regarding the early development of the egg. 



Metschnikow (41 1, in 1866, found that in Simulia the blastoderm 

 Covers the entire egg ; it is of the same thickness excepting at the 

 posterior pole where it is thicker. The cells of the blastoderm are 

 cylindrical, each containing a nncleus with a nucleole ; they lengthen 

 becoming of a different shape at the poles than those covering the 

 rest of the egg. In Cecidomyia , a Single nucleus was first noticed 

 within the egg; this divides and by repeated division and a wandering 

 of the nuclei, a zone of them is formed which finally passes to the 

 periphery where cells are formed. These are at first spherical, but 

 later become cylindrical. By viviparous aphids Metschnikow found 

 an early stage in the egg's development which contained but a Single 

 nucleus. This divides and the two resultant nuclei, at first near each 

 other in the center of the egg, separate; they divide, the division 

 continues and results in a layer of nuclei at the periphery. The 

 blastoderm cells form and lengthen, becoming largest at the broad 

 end of the egg. Metschnikow was against Weismann (59) in his 

 free cell formation theory, holding that the cleavage nuclei were 

 nuclei only, and that they gave rise to the nuclei of the blasto- 

 derm cells. 



Three years later Melnikow (40) observed in the eggs of 



