Identifier: physicalbasisofhmorg (find matches)
Title: The physical basis of heredity
Year: 1919 (1910s)
Authors: Morgan, Thomas Hunt, 1866-1945
Subjects: Chromosomes Heredity Chromosomes Genetics, Medical
Publisher: Philadelphia London : Lippincott
Contributing Library: Francis A. Countway Library of Medicine
Digitizing Sponsor: Open Knowledge Commons and Harvard Medical School

View Book Page: Book Viewer
About This Book: Catalog Entry
View All Images: All Images From Book
Click here to view book online to see this illustration in context in a browseable online version of this book.

Text Appearing Before Image:
ach half is also split lengthwise.A spindle appears, and the cells divide (Fig. 19, f,g,h).In each cell the chromosomes show indications of passinginto a resting stage, as happens after all ordinary celldivisions, but before this change has gone very far a newspindle appears (Fig. 19, i), and preparations for anotherdivision are rapidly made. The new division completesthe maturation of the sperm-cells (Fig. 19, j, kj). Eachof the four cells resulting from the original sperm-mother-cell differentiates into a spermatozoon. In one of the salamanders, Batrocoseps, the matura-tion stages of the male are particularly well shown. Theessential stages in synizesis are shown in Fig. 20, a-das worked out by Janssens. These stages are essentiallythe same as those of Tomopteris. During the early multi-plication stages the cells of the future testes divide by MECHANISM IN SEOREdATION 47 the ordinary mitotic process. The cells then pass into thesynaptic stage (Fig. 20, a-d). As the chromosomes begin
Text Appearing After Image:
FiQ. 19.—Thin-thread stage of Tomopteris spermatocyte, a-d; tetrads, e; first sperma-tocyte division, f-i; second spermatocyte division,;-^ (After Schreiner). to emerge as thin threads, it is found in Batracoseps thattheir ends are all pointed towards one pole (Fig. 20, d).This is the same pole as that towards which the two ends 48 PHYSICAL BASIS OF HEREDITY of each V-shaped chromosome pointed as the cell wentinto the resting stage. It appears> then that the chromo-somes not only retain their original orientation, but thatthe ends of homologous chromosomes have already come

Note About Images