Lab 2: Early Development in Sea Urchins


Sea Urchins have become a model species for the study of organism development due to the size of the female eggs, the availability of the organism and it’s gametes, and the cheap cost to keep the organism in a lab setting. In lab two we studied the gametes, anatomy, and development of the sea urchin using a microscope.
After measuring the diameter of ten unfertilized eggs with the 40X objective we got an average of 85.5 um ranging from 92.2 um to 74.24 um. On the other hand the average diameter of a fertilized was only 78.4 um smaller than a what was observed with the unfertilized egg. This measurement is odd because it goes against what I observed in the literature, lecture, and on the internet, the egg should have been of the same size, maybe even larger since the fertilization membrane should also be included. It is possible that the overall average of the unfertilized egg was larger than observed because there were two eggs found that were larger than 90um and these put the average high, and yet all the fertilized eggs fall within the range of what was recorded for the unfertilized eggs. Seamen size was averaged at 8.24 um but was very difficult to record due to the constant movement of the gametes and their small size.
The effect of time of sperm to fertilize an egg was observed over 7 intervals ranging from 0 sec age to 180 seconds. It was observed that over time the sperm’s ability to fertilize an egg declines and this must be due to the short life span of sperm which depend on the energy provided by the short supply of mitochondria. When added without aging the sperm had a fertilization percentage of 80% and over the seven intervals there was a constant decline that had 2% fertilization at 150 sec and a 0% fertilization at 180sec.
While observing the different stages of development from prepared slides we observed that after the first cleavage the overall size of the embryo does not change much till gastrulation where the embryo begins to “stretch”. Then once it reaches the larva stage there is a drastic change in morphology and the organism begins to grow in size. The most developed larva stage organism found was over 256um long. The final anatomical observation was the dissection of female and male sea urchins in where it was observed that both sea urchin sexes are morphologically identical except for the gametes they contain.
Lab 2 allowed us to observe sea urchins from the fertilization stage to the final reproductive organism.

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