Key points are not available for this paper at this time.
Abstract Counts have been made of the number of cells in the trochlear nucleus in a series of 22 normal chicks between the ninth day of incubation and the seventy‐fifth day after hatching, and in seven animals in which the optic vesicle and adjoining mesoderm were removed during the second day of incubation. Between the ninth and seventeenth days of incubation the number of cells in the normal trochlear nucleus declines from just under 1400 to a little under 700, indicating a cell loss of approximately 50% over this eight day interval. From the seventeenth day onwards there is a further slight fall to between 500 and 600 cells and this number is maintained into the third month after hatching. Following excision of the optic vesicle there is an earlier, and more severe, loss of neurons in the trochlear center, the number of cells being reduced to just under 1000 at the eleventh day of incubation and falling to just over 300 by the sixth day after hatching. Despite this marked cell loss, at no stage is there any gross evidence of neuronal disintegration or phagocytosis. The possible factors responsible for the normally occurring cell loss and for the hypoplasia in the trochlear nucleus after removal of the optic vesicle are discussed.
Cowan et al. (Wed,) studied this question.