THE SPERMATOGENESIS OF UMBRA LIMI WITH SPECIAL REFERENCE TO THE BEHAVIOR OF THE SPERMATOGONIAL CHROMOSOMES AND THE FIRST MATURATION DIVISION

¹ ZOÖLOGICAL LABORATORY, UNIVERSITY OF WISCONSIN

1. The spermatogonia of Umbra limi are formed anew each season from cells which migrate from some point outside the lobule, possibly in some cases even from outside the testis.

2. Evidence points to the presence of definite pro-chromosomes in the early spermatogonia.

3. The somatic and spermatogonial number of chromosomes is twenty-two. Chromosomes, at different times and in different cells, may exhibit contractions of varying degrees. Extreme contraction simulating a transverse fission is unusual.

4. Two large L-shaped chromosomes, relatively constant in size and shape, are found in all spermatogonial cells. They may be sex-chromosomes. If so, the male of Umbra is of the XX type.

5. The multiplication divisions of the spermatogonia are in general very rapid, with the result that the final spermatogonia are only of from one sixth to one third the volume of the original ones.

6. The primary spermatocytes of Umbra show roughly an increase in nuclear size of 15 to 20 per cent. over the spermatogonia of the preceding multiplication division.

7. The chromosomes emerge from synizesis in the form of rods or blocks of chromatin of varying length and thickness.

8. During diakenesis tetrads of the closed ring type appear. Final tetrad-formation is accomplished by the contraction of such rings. Carefully stained preparations show clearly the quadripartite nature of the tetrads in advanced metaphase when viewed from one pole.

9. Transversely constricted chromosomes are found in Umbra, but they are limited to certain spermatogonial divisions and play no part in the maturation divisions.

10. The tetrads have a median fiber attachment and separate into dyads at the metaphase of the first maturation division by a pulling apart at the apices of the loops or V's.

11. The secondary spermatocytes may go into a resting stage but usually the telophase of the primary spermatocyte passes immediately into the metaphase of the second maturation division. The separation of the dyads at the second maturation division is very rapid.

12. No unusually large or lagging chromosomes were observed in the first or second maturation divisions.

13. The nuclear material of the transforming spermatid goes through an apparent fusion of chromatin masses, with a subsequent eripheral condensation of the chromatin, and a contraction of the entire nucleus as the head of the spermatozoön forms.