i 9 o EMBRYOLOGICAL TYPES 



going to be males, these hollow out forming the seminiferous 

 tubules which become connected with the cavities of the tubules 

 of the mesonephros. In this way the vasa efferentia are formed, 

 and they may be regarded as persistent coelomic funnels, 

 placing the testis in communication with the exterior (via the 

 cloaca). The sperms therefore make their way through the 

 tubules of the mesonephros, down the Wolffian duct or vas 

 deferens as it can also be called, to the exterior. 



The Mullerian ducts develop as grooves in the roof of the 

 splanchnocoel at the side of the gonads. The sides of the 

 groove grow over, and convert it into a tube which opens into 

 the coelomic cavity in front (near the place where the pronephric 

 funnels were), and grows back to open into the cloaca behind. 

 In males the Mullerian ducts disappear. 



The kidneys and gonoducts are mesodermal all the way, 

 and are really ccelomoducts, whose primitive function is 

 probably to connect the coelomic cavity with the exterior and 

 so allow the germ-cells to escape. They take on the function 

 of excretion as a result of the proximity of the tubules to the 

 blood-vessels. 



On the other hand, the nephridia have excretion as their 

 primitive function ; they do not occur in Chordate animals 

 other than Amphioxus. 



Paired Sense-organs and Brain. — The eyes make their 

 appearance as outpushings from the sides of the brain, forming 

 the optic vesicles. Each of these vesicles grows towards the 

 overlying ectoderm, and becomes an optic cup, with the 

 concave side turned outwards. The lens is formed from the 

 ectoderm overlying the optic cup, as a little vesicle which soon 

 becomes nipped off, and sinks into place at the mouth of the 

 cup. While the cup is really part of the brain, the lens is 

 part of the epidermis, but both are ectodermal. The outer 

 lining of the cup forms the pigment or tapetum layer, the 

 inner lining of the cup differentiates to form the sensitive 

 retina, and it is inverted since the nerve-fibres run between 

 the sensitive cells and the seen object (see p. 24). Outside 

 the tapetum, mesodermal tissue gives rise to the choroid and 

 sclerotic (including the transparent cornea) layers, just as round 



