  Star formation within the Central Molecular Zone (CMZ) may be intimately
linked to the orbital dynamics of the gas. Recent models suggest that star
formation within the dust ridge molecular clouds (from G0.253+0.016 to Sgr B2)
follows an evolutionary time sequence, triggered by tidal compression during
their preceding pericentre passage. Given that these clouds are the most likely
precursors to a generation of massive stars and extreme star clusters, this
scenario would have profound implications for constraining the time-evolution
of star formation. In this Letter, we search for the initial conditions of the
protocluster clouds, focusing on the kinematics of gas situated upstream from
pericentre. We observe a highly-regular corrugated velocity field in
$\{l,\,v_{\rm LSR}\}$ space, with amplitude and wavelength $A=3.7\,\pm\,0.1$
kms$^{-1}$ and $\lambda_{\rm vel, i}=22.5\,\pm\,0.1$ pc, respectively. The
extremes in velocity correlate with a series of massive
($\sim10^{4}$M$_{\odot}$) and compact ($R_{\rm eq}\sim2$ pc), quasi-regularly
spaced ($\sim8$ pc), molecular clouds. The corrugation wavelength and cloud
separation closely agree with the predicted Toomre ($\sim17$ pc) and Jeans
($\sim6$ pc) lengths, respectively. We conclude that gravitational
instabilities are driving the condensation of molecular clouds within the
Galactic Centre gas stream. Furthermore, we speculate these seeds are the
historical analogue of the dust-ridge molecular clouds, representing the
initial conditions of star and cluster formation in the CMZ.
