Зоология на английском. Обязательный материал для чтения на занятиях и дома. Сологуб Л.И. - 15 стр.

UptoLike

ВУЗ: 

СГАУ | Самара

Составители: 

Рубрика: 

V. Explain the following terms: ectoderm, endoderm. Then let your groupmate
answer your questions on the two methods of reproduction of hydra. Begin your
questions with: I wonder... I want to know... I'd like to know...
VI. Make up a model of the text, sum up its information up.
Text 8
I. Read the text a say what it is about.
FERTILIZATION
Fertilization is the entry of a sperm into the egg and the fusion of their two
nuclei. In hydra many thousands of sperms are set free from the tester into the
water and they swim about for a short time. If some come near to an individual
whose ovary contains a ripe egg they will collect round it, and one sperm will
enter and fertilize it. ?or reasons which we do not understand, this fusion of
nuclei stimulates the egg to develop into a new individual, i.e. it makes the egg
fertile. The very large number of sperms will help to prevent eggs remaining
unfertilized.
The fertilized egg cell does not form a new hydra directly. It divides to form
two cells, then again to produce four, and eight, and sixteen, and so on, until an
embryo of many small cells has been formed. This makes a horny case for itself
before it detaches from the parent to remain in the mud for several months. Later
a snail hydra emerges from the case, starts to feed and grow, and then forma
buds if food is plentiful.
Organisms are male if they produce sperms and female if they produce-
eggs. Hydra can often produce both, although not at the same time. Such
organisms may be called hermaphrodite. Earthworms are also hermaphrodite
and, like hydra, do not fertilize eggs with their own sperms.
The methods which involve the separation of a part of the parent which
could itself develop into a new individual without any fusion or fertilization are
examples of asexual reproduction.
II. Remember the following words, consult a dictionary about the way they are
pronounced:
fusion слияние
unfertilized неоплодотворенный
embryo эмбрион, зародыш
horny case изгиб в форме рога
hermaphrodite обоеполое существо
14

Страницы