86 NATURAL SCIENCE. Feb., 



Ligia, and Porcellio, and again we find exactly the same number in 

 Mysis:' 



Side by side with the more or less exact similarity of the 

 appendages, Mr. Groom shows that there are numerous differences 

 in other parts which distinguish the various cirripede Nauplii, though 

 not in all cases very conspicuously, for those of Conchoderma virgata 

 and Lepas anatifem can only be known apart by the slight fork in the 

 tail of the latter. Of these two species the ova and embryos are 

 almost indistinguishable. As a rule, however, differences between 

 species go back as far as the new-laid ovum, affecting its size, shape, 

 colour, and constitution. In Balamis the ova are brown, in Chtha- 

 malns orange-coloured, in Dichelaspis vermilion-red, in Lepas and 

 Conchoderma blue. But the observations only refer to a limited 

 number of species, and in Lepas anatifeva, for example, "the blue 

 colour becomes in older embryos replaced by red." 



According to Mr. Groom, when the cirripede egg has been laid, 

 a small mass of clear protoplasm is constricted off from it, known as 

 the first polar body, the external hyahne layer of protoplasm meanwhile 

 secreting a delicate firm pellicle — the vitelline or perivitelline mem- 

 brane. Then, it is supposed, fertilisation takes place, the ova without 

 this undergoing no further changes. But " the entrance of the 

 spermatozoon has never been witnessed in Cirripedes." After ferti- 

 lisation, the contents of the egg contract considerably, the protoplasm 

 undergoes marked rhythmical contractions, the clear area at the 

 anterior end becomes amoeboid, and throws out short, blunt pseudo- 

 podia, which are often retracted. Then the second polar body is 

 constricted off. 



Here it may be noticed that in the isopods Dr. McMurrich 

 places fertilisation before, instead of after, the formation of the vitelline 

 membrane, for which, he says, "the primary condition is understood 

 more especially through the observations of Fol and the Hertwigs 

 (1887) to be normally a stimulus imparted to the egg protoplasm by 

 the spermatozoon." 



To return to the cirripede ovum. By degrees the protoplasm is 

 mainly collected at the anterior pole and the yolk at the other. The 

 yolk at this stage, Mr. Groom says, is devoid of a special nucleus, and, 

 contrary to the general opinion, "is in no way comparable to an 

 endoderm cell." He rejects Nussbaum's theory of a rotation of the 

 plane which separates the protoplasm from the yolk. " The shape of 

 the protoplasmic portion of the egg at this stage is generally ovoid, a 

 central plug of greater or less extent reaching into the middle of the 

 yolk, which thus fits like a thick-bottomed bowl on to the central 

 mass." Another point on which Mr. Groom insists is that " the 

 protoplasm of the first blastomere gives rise to a portion only of 

 the ectoderm," and that " the second blastomere does not come from 

 the first, hxii from the yolk.'' 



When he comes to discuss the formation of the Nauplius seg- 



