Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!csd4.milw.wisc.edu!lll-winken!uunet!mcvax!cernvax!ascom!rene From: rene@ascom.uucp (Rene Bach) Newsgroups: sci.bio Subject: Re: embryology. A mini tutorial Message-ID: <169@ascom.UUCP> Date: 20 Apr 89 07:33:16 GMT References: <3229@imagen.UUCP> Reply-To: rene@ascom.UUCP (Rene Bach) Organization: Ascom Tech AG, Solothurn, CH Lines: 22 Unfortunately, I haven't studied biology in english, so there might be some semantic misunderstanding in the previous message. Sexual cells, i.e. those that will merge to generate a new individual have undergone MEIOSIS. They have lost their second set of chromosomes. They only have 23 (for humans). Sexual cells are the sperm cells and the ovae (ovum). Checking in the dictionary, it appears that the latter are also called EGGS. To make sure, call it the UNFERTILIZED EGG, then it definitely (if it is normal) has only 23 chromosomes (for humans). Upon fertilization, the two sex cells merge and undergo a process of chromosome grouping into a nucleus. This is the fertilized egg or embryo. It will undergo many, many cell divisions using MITOSIS. somatic cells undergo MITOSIS (duplication of chromosomes and even distribution in the two daughter cells) and if normal, have a double set of chromosomes (i.e. pairs, i.e. 2x 23 = 46). The only pair of chromosomes that look different is the SEX chromosome pair. XY = males and XX for females (valid in many species but I believe not ALL). Rene Bach Switzerland