python - scrapy 回调函数使用？

请问一下 ，

class myspider(scrapy.Spider):

name = 'novel'
allowed_domains = ['23wx.com']
bash_url = 'http://www.23wx.com/class/'
bashurl = '.html'

def start_requests(self):
    for i in range(1, 11):
        url = self.bash_url + str(i) + '_1' + self.bashurl
        yield Request(url, self.getallurl)

def getallurl(self, response):
    print response.body

上面这个能正确输出html内容

但是下面这个却不能输出

class myspider(scrapy.Spider):

name = 'novel'
allowed_domain = ['23wx.com']
def get_start_url(self):
    for i in range(1,11):
        url  = 'http://www.23wx.com/class/{}_1.html'.format(i)
        yield Request(url,callback=self.getallurl)

def getallurl(self,response):
    print response.body

实在不清楚两者之间有什么区别，还是回调函数的使用有什么注意的地方。求指教