Delayed Implantation and Discontinuous Development in the Mammals
| Autor: | G. W. D. Hamlett |
|---|---|
| Rok vydání: | 1935 |
| Předmět: | |
| Zdroj: | The Quarterly Review of Biology. 10:432-447 |
| ISSN: | 1539-7718 0033-5770 |
| DOI: | 10.1086/394493 |
| Popis: | JT HAS long been a matter of general knowledge that the rate of development of the cold-blooded vertebrates is highly variable; the rate of growth of a frog embryo, for example, being almost as exact a function of the temperature as is the speed of a chemical reaction. Even the birds have retained to some degree this power of slowing or temporarily suspending growth for the several hours or days intervening between laying and incubation. It seems, however, to be generally believed that the mammalian ovum, with its intrauterine development at practically constant temperature, has gotten completely away from the archaic habit of hesitating in its growth. At least the idea that a mammalian embryo may suddenly come to a dead stop in its development and remain quiescent for weeks or months before resuming its growth would probably come as a shock to many biologists who are not familiar with embryological literature. Yet we know at present of a dozen mammals whose period of gestation is regularly from two to six months longer than actual embryonic development, and in six of these species we have definite embryological proof that this lengthening of pregnancy is due to a quiescent period interpolated in the development. In addition to this group of species in which the quiescent period is a normal feature of development, there are several species of rodents in which development has been experimentally slowed down or temporarily stopped by various means; but at present it is uncertain as to whether this ever occurs except under the artificial conditions of the laboratory. In spite of its unexpectedly frequent occurrence, the fact that mammalian development may be even more discontinuous than that of bird, frog, or fish has scarcely found its way into the text or reference books; certainly none of them gives any idea of the frequency with which this phenomenon is found. This cannot be blamed on any recency of the work, for seven of the cases have been known for more than forty years. It is not only the writers of texts who have overlooked this field; some of the investigators who have worked in it have been almost as negligent in ignoring the work done in other countries or on other forms. In the following pages I wish to summarize our present knowledge of the subject in the hope of making the facts more generally known and of stimulating additional work in the field. |
| Databáze: | OpenAIRE |
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