问题 问答题 简答题

动物由囊胚发育形成原肠胚的主要形式有哪些?

答案

参考答案:

动物由囊胚发育形成原肠胚的主要形式有5种,分别是:

内陷:由囊胚的植物极细胞向内陷入,最后形成2层细胞,在外面的细胞层称外胚层,内层细胞称内胚层。由内胚层所围成的腔称原肠腔,它的开口称胚孔或原口。

内移:是由囊胚一部分细胞移入内部形成内胚层。

分层:囊胚的细胞分裂时,细胞沿切线方向分裂,这样向着囊胚腔分裂出的细胞为内胚层,外面的细胞层形成外胚层。

内转:通过盘裂形成的囊胚,分裂的细胞由下面边缘向内转,伸展形成内胚层。

外包:动物极细胞分裂快,植物极细胞由于卵黄多而分裂慢,结果动物极细胞逐渐向下包围植物极细胞,形成内外胚层。

但在动物实际的胚胎发育中,通常是其中的几种方式混合发生。

单项选择题
阅读理解

I love secondhand books far more than new ones! I especially like books that someone else has gone through first. It’s a kind of connection — and an experience of wondering exactly who has been there before…I don’t mind god-eared (折角的)corners;I don’t mind the coffee stain—who cares? Indeed, I don’t even mind the comments at the edge.  

If it were an orginal classic, then the book would be of some importance. But the fact that its pages have been read and probably loved by dozens of people since it was produced is incredible, fascinating and wonderful. Just think — you can find old books that were touched by people in the nineteen fifties. That is interesting, as those people would probably have just come through World War Ⅱ.

But books from the 20s and 30s are even more wonderful to touch. Was it an old lady who poured over my aging Dodi Smith paperback that I found in a secondhand bookshop?

I once sat in the British Library reading books published in the 17th century, touching them, and the excitement was considerable. Imagine — these are books that people read, touched , and wrote comments about hundreds of years ago. Who have touched these books? What were they wearing? What was their work? How did the books make them think?

Now you see— I am just a woman who likes to explore the past. I like anything except the new. Who wants new things? The smell of the chemicals, the sense of being mass-produced…

No! No!

46. We can know that when the writer reads a second-hand book, she usually__________.

A. cleans the coffee stain         B. writes comments at the edge

C. wonders who has read it before  D. only reads the comments written by others

47. According to Paragraph 2, what does the writer find fascinating?

A. The economic value of an original classic.

B. The fact that many old books are about World War Ⅱ.

C. The fact that many people like reading what she likes reading.

D. The fact that a book has been read and loved by many people.

48. The underlined part “Dodi Smith paperback” in Paragraph 3 refers to “ ______”.

A. an old lady         B. a book by Dodi Smith

C. a famous writer      D. a secondhand bookshop

49. After reading the passage, we know that the writer __________.

A. only buys books with dog-eared corners

B. seldom looks back on her past

C. likes the smell of new books

D. doesn’t like things mass-produced today 

50. What is this passage mainly about?

A. The advantages of reading old books.

B. Some of the writer’s strange hobbies.

C. The writer’s love for secondhand books.

D. What the writer does in her spare time.