I06 PRINCIPLES OF EMBRYOLOGY 



vesicle, with a diameter up to half that of the egg. It lies at one side, closely 

 against a peripheral layer of clear cytoplasm which encloses the main 

 bulk of the egg. Conklin points out that in Amphioxus, and probably in 

 ascidians, the attachment of the egg to the wall of the ovary is by the end 

 containing the germinal vesicle, that is by the animal pole, whereas in 

 most invertebrates it is the vegetative pole which is attached; this differ- 

 ence he correlates with the fact that the vertebrates have a dorsal nerve 

 cord and the invertebrates a ventral one, so that the two kingdoms seem 

 to differ by a reversal of their dorso-ventral axis. The suggestion is an 

 interesting one, but it is by no means clear that most vertebrate eggs are 

 like Amphioxus in having their animal pole attached to the ovary wall. In 

 the frog, the point of attachment seems to be slightly below the equator. 

 Both ascidians and Amphioxus provide beautiful examples of the 

 important fact that an egg is not a mere lump of featureless cytoplasm 

 furnished with a haploid nucleus, but on the contrary has an effective archi- 

 tecture of its own. The eggs of some ascidians, such as Styela, described 

 by Conklin, are perhaps the clearest instances of this to be found in the 

 animal kingdom, since in this form several different regions of cytoplasm 

 can be visibly distinguished owing to their content of coloured granules, 

 mitochondria, yolk, etc. Before fertilisation, the egg of Styela has a 

 peripheral layer of clear yellowish cytoplasm, inside which is a grey 

 yolk-laden cytoplasm, and the large geriTiinal vesicle filled with clear 

 sap (Fig. 7.1). The sperm enters always near the vegetative pole. As it 

 penetrates it sets off a reaction of the egg surface, which in this instance 

 involves a streaming of the yellow peripheral cytoplasm downwards to 

 the vegetative pole. Simultaneously the germinal vesicle undergoes the 

 reduction divisions and breaks down to give rise to the polar bodies; the 

 clear plasm thus released also moves down the egg and lies above the 

 yellow material just above the vegetative region. The sperm head now 

 starts to move upwards, remaining fairly near the egg surface. As it does 

 so, it appears to pull the two layers of clear and yellowish cytoplasm 

 along with it, the yellowish material remaining on the surface, wliile the 

 colourless extends deep into the centre of the egg. After throwing off the 

 polar bodies, the egg nucleus descends towards the sperm nucleus, and 

 they meet just below the equator but away to the side of the egg towards 

 which the sperm has been travelling. Since the yellow and colourless 

 materials have been accompanying the sperm, they are now seen as two 

 crescents, clear above and yellow below, with their thickest part in the 

 meridian along which the sperm moved. This meridian is clearly a plane 

 of bilateral symmetry in the egg. The first cleavage plane falls along it, 

 and in the final embryo it is the plane dividing the right side from the 



